Electricity accumulating device

ABSTRACT

An electricity accumulating device includes capacitors connected in series, balanced voltage adjusting portions connected to the capacitors respectively, and a control circuit connected to the balanced voltage adjusting portions. The control circuit performs the following operations: measuring two voltages at different times from each other across each capacitor during the non-charge-or-discharge period of the capacitors by using the balanced voltage adjusting portions; calculating the absolute value of the difference between the two voltages; determining the balanced voltage of each of the capacitors according to the absolute value; and controlling the balanced voltage adjusting portions to make the voltage across each capacitor a balanced voltage.

THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCTINTERNATIONAL APPLICATION PCT/JP2009/000004.

TECHNICAL FIELD

The present invention relates to an electricity accumulating devicehaving capacitors for storing and discharging electric power.

BACKGROUND ART

With increasing concern for the environment in recent years, electricand hybrid cars, which are wholly or partially driven by a motor, aregrowing in popularity. In these cars (hereinafter, vehicles), theelectric power of the motor is supplied from a battery. Such batteries,however, are likely to change their characteristics or to be degradedwhile being rapidly charged or discharged with a large current. To avoidthis, restrictions are imposed on the current supplied to the motor whenthe driver tries to accelerate too fast. This, however, sometimes makesacceleration insufficient.

To overcome this problem, there have been proposed vehicles having botha battery and capacitors with rapid charging-dischargingcharacteristics. In such vehicles, the motor is supplied with electricpower from both the battery and the capacitors when the driver tries toaccelerate too fast. As a result, the vehicle can be accelerated fasterthan in the case of having a battery only.

To obtain a voltage high enough to drive a motor from capacitors,assuming that the voltage is about 750V, it is necessary to connect 300capacitors each having a rated voltage of 2.5V in series. There are alsocases in which some capacitors are connected in series and the othersare connected in parallel to provide a necessary capacitance.

Capacitors, however, have variations in characteristics, and therefore,are applied with different voltages from each other. If charged withoutconsidering this, capacitors may be degraded, thereby shortening theirlife.

Under such circumstances, there have been proposed electricityaccumulating devices in which a large number of capacitors have smallvariations in the degree of degradation, and hence, a long life.

FIG. 13 is a block circuit diagram of a conventional charging device. Asshown in FIG. 13, the electricity accumulating device includescapacitors 501 connected in series, and balanced voltage adjustingportions 503 connected to capacitors 501 at their both ends. Eachcapacitor 501 is also connected at its both ends via two switches 507 toa corresponding sampling capacitor 505 for measuring the voltage acrossthe corresponding capacitor 501. Balanced voltage adjusting portions 503and switches 507 are connected to controller 509. Although it is notillustrated, capacitors 501 connected in series are also connected tothe motor, the generator, the battery, the loads, and other componentsof the vehicle via charge-discharge circuits.

Each balanced voltage adjusting portion 503 includes a series circuitconnected to both ends of the corresponding capacitor 501. The seriescircuit includes balance switch 511 and balance resistor 513. Eachbalanced voltage adjusting portion 503 also includes two partialpressure resistors 515 connected in series, which are also connected toboth ends of the corresponding capacitor 501. The connection point oftwo partial pressure resistors 515 is connected to one input ofcomparator 517. The other input of comparator 517 is connected todigital potentiometer 519. Digital potentiometer 519 is connected toreference supply 521 and controller 509 so as to output a referencevoltage in accordance with the instructions from controller 509. Theoutput of comparator 517 is connected to balance switch 511 so as tocontrol its on-off operation.

This electricity accumulating device operates as follows. First,controller 509 calculates the degree of degradation of each capacitor501. More specifically, controller 509 calculates a capacitance C fromthe gradient in the change of the voltage across each capacitor 501 whenit is charged at a constant current, and also calculates an internalresistance R from the change in the voltage across each capacitor 501when the charge is suspended. Controller 509 then calculates thedifferences between the capacitance C and its predetermined degradationlimit and between the internal resistance R and its predetermineddegradation limit, thereby determining the degree of degradation fromthe differences. Therefore, the smaller the differences, the greater thedegree of degradation.

Next, controller 509 calculates the average of the degree of degradationof all capacitors 501, and determines the balanced voltage of eachcapacitor 501 in such a manner as to reduce variations in the degree ofdegradation in all capacitors 501. In the case of a highly degradedcapacitor 501, controller 509 determines a balanced voltage that reducesthe voltage across the capacitor 501 so as to delay the degradation.Then, controller 509 controls each balanced voltage adjusting portion503 to make the voltage across each capacitor 501 the balanced voltage.

Thus, the balanced voltages of capacitors 501 are adjusted so as toreduce variations in the degree of degradation in all capacitors 501.This delays the degradation of highly degraded capacitors 501, therebyallowing all capacitors to reach operating limits substantially at thesame time. As a result, the electricity accumulating device has a longlife. This technique is disclosed in Patent Literature 1.

The above-described conventional electricity accumulating device,however, is required to perform the following complex operations whilecapacitors 501 are being charged with a constant current. First, thecapacitance C and the internal resistance R of each capacitor 501 arecalculated. Next, the degree of degradation of each capacitor 501 iscalculated from them. Then, the balanced voltage of each capacitor 501is calculated from the average of the degree of degradation of allcapacitors 501 in such a manner as to reduce variations in the degree ofdegradation in all capacitors 501.

Patent Literature 1: Japanese Patent Unexamined Publication No.2007-124883

SUMMARY OF THE INVENTION

The present invention provides an electricity accumulating device havingcapacitors whose life is extended accurately by a simple operation.

The electricity accumulating device of the present invention includes aplurality of capacitors connected in series, a plurality of balancedvoltage adjusting portions connected to the capacitors respectively, anda control circuit connected to the balanced voltage adjusting portions.The control circuit performs the following operations: measuring the twovoltages (V1 i and V2 i, where “i” is 1 to “n”, where “n” represents thenumber of the capacitors) at different times from each other across eachof the capacitors during the non-charge-or-discharge period of thecapacitors by using the balanced voltage adjusting portions; calculatingthe absolute value (ΔVi) of the difference between the voltages (V1 iand V2 i) across each of the capacitors; measuring a first point of time(t1) at which the voltage (V1 i) is measured and a second point of time(t2) at which the voltage (V2 i) is measured; calculating the timedifference (Δt) by subtracting the first point of time (t1) from thesecond point of time (t2); calculating a voltage adjustment range (ΔVbi)of each of the capacitors by dividing the absolute value (ΔVi) by thetime difference (Δt) and multiplying the result by a specifiedcoefficient (A); determining the balanced voltage (Vri) by subtractingthe voltage adjustment range (ΔVbi) from an initial balanced voltage(Vro); and controlling the balanced voltage adjusting portions to make avoltage (Vi) across each of the capacitors the balanced voltage (Vri).

According to the electricity accumulating device of the presentinvention, the control circuit calculates the absolute value (ΔVi) ofthe difference between the two voltages (V1 i and V2 i) measured atdifferent times from each other across each of the capacitors, anddetermines the balanced voltage (Vri) therefrom while the capacitors arenot being charged or discharged. This eliminates the need to calculatethe capacitance C or the internal resistance R while the capacitors arebeing charged at a constant current as in the conventional electricityaccumulating devices. This also eliminates the need to control thedetermination of the balanced voltage from the average of the degree ofdegradation. As a result, the life of the capacitors can be extended bya simpler operation than in the conventional electricity accumulatingdevices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block circuit diagram of an electricity accumulating deviceaccording to a first exemplary embodiment of the present invention.

FIG. 2 is a graph showing the change in the voltage across eachcapacitor from time t1 to time t2 in the electricity accumulating deviceaccording to the first exemplary embodiment of the present invention.

FIG. 3 is a flowchart showing a process for determining the balancedvoltage of each capacitor in the electricity accumulating deviceaccording to the first exemplary embodiment of the present invention.

FIG. 4 is a flowchart showing a process for determining the balancedvoltage of each capacitor in an electricity accumulating deviceaccording to a second exemplary embodiment of the present invention.

FIG. 5 is a graph showing the change in the voltage across eachcapacitor from time t1 to time t2 in an electricity accumulating deviceaccording to a third exemplary embodiment of the present invention.

FIG. 6 is a graph showing the change in the characteristics with time ofthe full voltage of the electricity accumulating device according to thethird exemplary embodiment of the present invention.

FIG. 7 is a flowchart showing a process for determining anon-charge-or-discharge-period voltage and a charge-or-discharge-periodvoltage across each capacitor in the electricity accumulating deviceaccording to the third exemplary embodiment of the present invention.

FIG. 8 is a flowchart showing a process for determining the balancedvoltage of each capacitor in the electricity accumulating deviceaccording to the third exemplary embodiment of the present invention.

FIG. 9 is a flowchart showing a process for determining the balancedvoltage of each capacitor in an electricity accumulating deviceaccording to a fourth exemplary embodiment of the present invention.

FIG. 10 is a graph showing the change in the voltage across eachcapacitor from time t1 to time t2 in an electricity accumulating deviceaccording to a fifth exemplary embodiment of the present invention.

FIG. 11 is a flowchart showing a process for determining the balancedvoltage of each capacitor in the electricity accumulating deviceaccording to the fifth exemplary embodiment of the present invention.

FIG. 12 is a flowchart showing a process for determining the balancedvoltage of each capacitor in an electricity accumulating deviceaccording to a sixth exemplary embodiment of the present invention.

FIG. 13 is a block circuit diagram of a conventional electricityaccumulating device.

REFERENCE MARKS IN THE DRAWINGS

-   11 capacitor-   13 balanced voltage adjusting portion-   15 control circuit-   25 temperature sensor

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiments for carrying out the present invention will bedescribed by means of embodiments with reference to accompanieddrawings. In the embodiments, the electricity accumulating device isused in a hybrid vehicle.

First Exemplary Embodiment

FIG. 1 is a block circuit diagram of an electricity accumulating deviceaccording to a first exemplary embodiment of the present invention. Asshown in FIG. 1, the electricity accumulating device includes capacitors11 connected in series. In the drawing, thick lines represent electricalwires, and thin lines represent signal wires. Capacitors 11 in thepresent first exemplary embodiment are electric double layer capacitorshaving a large capacitance. Capacitors 11 may alternatively be connectedin series-parallel according to the power specification required. Inthis case, capacitors connected in parallel are treated as one capacitor11 so as to make them have an equivalent circuit of capacitors 11 shownin FIG. 1. Therefore, the following description is based on theassumption that each capacitor 11 can be either one or a plurality ofcapacitors connected in parallel. It is possible that the end terminalsof series-connected capacitors 11 in the electricity accumulating deviceof FIG. 1 are connected to the end terminals in other electricityaccumulating devices. In such series-parallel connection, each capacitor11 is connected to the corresponding balanced voltage adjusting portion13, which will be described later.

Capacitors 11 are connected at their both ends to balanced voltageadjusting portions 13. Balanced voltage adjusting portions 13 areconnected to control circuit 15, which controls the operation ofbalanced voltage adjusting portions 13. Control circuit 15 is composedof peripheral circuits and a microcomputer for controlling them. Theperipheral circuits have functions of, for example, digitalpotentiometer 519 and reference supply 521 of the conventionalelectricity accumulating device shown in FIG. 13. Control circuit 15also has a function of exchanging data with a vehicular control circuit(not shown) by using a data signal (Data).

The following is a description of the structure of balanced voltageadjusting portions 13. Each balanced voltage adjusting portion 13includes a series circuit connected to both ends of capacitor 11. Theseries circuit includes balance switch 17 and balance resistor 19.Balance switch 17 can be externally on-off controlled, and can be, forexample, a FET or a transistor. Each balanced voltage adjusting portion13 includes another series circuit connected to both ends of thecorresponding capacitor 11. The series circuit has two partial pressureresistors 21. The connection point of two partial pressure resistors 21is connected to control circuit 15, and also to one input of comparator23. Therefore, control circuit 15 can read a voltage Vi (i=1 to n, wheren represents the number of capacitors 11 connected in series) acrosseach capacitor 11. The positive electrode of the uppermost capacitor 11shown in FIG. 1 has the same voltage as a full voltage Vc of allcapacitors 11 connected in series. Therefore, control circuit 15 canalso read the full voltage Vc via the uppermost balanced voltageadjusting portion 13.

The other input of comparator 23 is connected to control circuit 15, sothat comparator 23 can receive a balanced voltage Vri from controlcircuit 15. The output of comparator 23 is connected to balance switch17 so as to control the on-off operation of balance switch 17.

The electricity accumulating device includes temperature sensors 25 nearcapacitors 11. Temperature sensors 25 are thermistors having a largechange in resistance with temperature. The outputs of temperaturesensors 25 are connected to control circuit 15, so that control circuit15 can read a temperature T detected by temperature sensor 25.

Positive terminal 27 and negative terminal 29, which are the endterminals of series-connected capacitors 11, are connected to the motor,the generator, the battery, the loads, and other components of thevehicle via charge-discharge circuits. These components are notillustrated in FIG. 1.

The electricity accumulating device having the above-described structureoperates as follows.

FIG. 2 is a graph showing the change in the voltage across eachcapacitor from time t1 to time t2 in the electricity accumulatingdevice. In FIG. 2, the horizontal axis represents the time “t”, and thevertical axis represents the voltage Vi across each capacitor 11. In thecase of a hybrid vehicle, several hundred capacitors 11 are connected inseries as mentioned above, however, in the following description, onlyfour capacitors 11 are connected in series for easier explanation. Thus,the number “n” of capacitors 11 is 4, and the subscript “i” is in therange of 1 to 4.

First assume that at the time t1, the ignition switch (not shown) of thevehicle is turned on and the vehicle is started. Control circuit 15recognizes the start of the vehicle when receiving a signal indicatingthat the ignition switch has been turned on as a data signal (Data) fromthe vehicular control circuit. Control circuit 15 may alternativelyrecognize the start of the vehicle by a driving voltage which issupposed to be applied to control circuit 15 when the ignition switch isturned on. Upon recognizing the start of the vehicle, control circuit 15reads the present voltage V1 i (i=1 to 4) across each capacitor 11sequentially from balanced voltage adjusting portions 13, and storesthem in a memory embedded in control circuit 15. At the same time,control circuit 15 stores the time t1 as a first point of time t1 in thememory. This means that the first point of time t1 has been measured.Capacitors 11 have respective voltages V11 to V14 across themselves,which are in a low state due to self-discharge until the time t1 afterthe use of the vehicle is finished last time. The voltages V11 to V14are different from each other due to variations in the characteristicsand the degree of degradation in all capacitors 11.

The next time the vehicle is in use, capacitors 11 are charged withregenerative electric power whenever the brakes are applied, and thestored electric power is discharged therefrom whenever the vehicle isaccelerated. This change with time is not shown in FIG. 2.

Next assume that the use of the vehicle is finished at the time t2.Since the use of the vehicle is generally finished when the brakes areapplied and then the vehicle is brought to a halt, capacitors 11 arecharged with the regenerative electric power generated at the time ofbraking. Therefore, at the time t2 of FIG. 2, each capacitor 11 has avoltage V2 i (i=1 to 4) across themselves, which is larger than thevoltage V1 i at the time t1. Control circuit 15 reads voltages V21 toV24 across respective capacitors 11 at the time t2 sequentially frombalanced voltage adjusting portions 13, and stores them in the memory.Control circuit 15 also stores the time t2 as a second point of time t2.This means that the second point of time t2 has been measured.

As is obvious from the above description, both the times t1 and t2 arein the non-charge-or-discharge period in which capacitors 11 are notbeing charged or discharged. This allows the voltages V1 i and V2 iacross each capacitor 11 to be measured in a stable state. The term“non-charge-or-discharge period” is defined as a period in whichcapacitors 11 are not being aggressively charged or discharged by thecharge-discharge circuits (not shown). In other words, thenon-charge-or-discharge period includes not only a state in which nocurrent is being supplied to capacitors 11, but also a state in which aslight leakage current is flowing to capacitors 11 although thecharge-discharge circuits are not being operated.

The voltage Vi across each capacitor 11 changes with temperature, andtherefore, control circuit 15 stores the predetermined temperaturedependence of the voltage Vi. Control circuit 15 then corrects thevoltages V1 i and V2 i across each capacitor 11, based on the storedtemperature dependence according to the temperature T from temperaturesensor 25.

More specifically, control circuit 15 determines the temperaturedependence of the voltage Vi across each capacitor 11 when thetemperature T is changed after capacitors 11 have been charged to aknown voltage at a reference temperature To (for example, 25° C.).Control circuit 15 determines the temperature dependence in every knownvoltage range (for example, 0.1V) until the known voltage reaches arated voltage (for example, 2.5V) of capacitors 11. The temperaturedependence of the voltage Vi across each capacitor 11 is repeatedlydetermined until capacitors 11 reach the rated voltage (2.5V) whilechanging the temperature T. The temperature T can be changed at thefollowing times: when capacitors 11 have been charged first to 0.1V,next to 0.2V, then to 0.3V, and so on at the reference temperature To(25° C.). The temperature dependences thus obtained are previouslystored in the memory of control circuit 15.

When the temperature T and a voltage Vi across capacitor 11 are thusobtained, control circuit 15 selects the temperature dependencecorresponding to the voltage Vi at the temperature T from among thetemperature dependences. Then, control circuit 15 determines the voltageVi at the reference temperature To, based on the selected temperaturedependence. The voltage Vi thus determined becomes atemperature-corrected value.

Therefore, even when the temperatures are different at the times t1 andt2 of FIG. 2, the voltage Vi across capacitor 11 at the temperature T iscorrected to the voltage Vi across capacitor 11 at the referencetemperature To, thereby improving the accuracy of calculating thebalanced voltage Vri, which will be described below. The temperaturecorrection can reduce the speed of the degradation of capacitors 11(which will be described in detail later) with high accuracy,contributing to the long life of capacitors 11.

The magnitude relation between the voltages V21 to V24 across respectivecapacitors 11 is not necessarily the same as the magnitude relationbetween the voltages V11 to V14 across respective capacitors 11, but maybe the other way around according to the variations in thecharacteristics or the degree of degradation in all capacitors 11. Morespecifically, in FIG. 2, capacitor 11 having the largest voltage V11across itself at the time t1 has the smallest voltage V21 at the timet2, and capacitor 11 having the smallest voltage V14 across itself atthe time t1 has the largest voltage V24 at the time t2. Therefore, inthe present first exemplary embodiment, the balanced voltage Vri of eachcapacitor 11 is determined based on the inclination of each thick arrowshown in FIG. 2, which is obtained from the voltages V1 i and V2 imeasured at the times t1 and t2 across each capacitor 11.

The following is a specific description of how the balanced voltage Vriis determined.

FIG. 3 is a flowchart showing a process for determining the balancedvoltage of each capacitor in the electricity accumulating deviceaccording to the first exemplary embodiment. The flowchart of FIG. 3 isshown as a subroutine because control circuit 15 controls the operationof the electricity accumulating device as a whole by executing differentsubroutines called from the main routine.

Control circuit 15 executes the subroutine of FIG. 3 at the time t2 whenthe first and second points of time t1 and t2, and the voltages V1 i andV2 i across each capacitor 11 are all obtained. First, control circuit15 assigns 1 to a variable memory “i” embedded therein (step number:S11). The variable memory “i” is defined to have the same meaning as,and is hereinafter referred to as the subscript “i”. Step S11 in theflowchart shows “i=1”, which is defined to mean that the value on theright-hand side is assigned to the variable on the left-hand side in thefollowing description. Therefore, in S11, “1”, which is the value on theright-hand side is assigned to the subscript “i”, which is the variableon the left-hand side.

Next, control circuit 15 calculates the absolute value ΔVi of thedifference between the voltages V1 i and V2 i across each capacitor 11from an equation: ΔVi=|V2 i−V1 i| (S13). Then, control circuit 15calculates a time difference Δt by subtracting the first point of timet1 from the second point of time t2 (S14). In other words, controlcircuit 15 calculates the time difference Δt from an equation: Δt=t2−t1.Next, control circuit 15 calculates a voltage adjustment range ΔVbi ofeach capacitor 11 from the absolute value ΔVi, the time difference Δt,and a specified coefficient “A”, based on an equation: ΔVbi=A×ΔVi/Δt(S15), where ΔVi/Δt is the inclination of each thick arrow in FIG. 2.Each inclination corresponds to the reciprocal of the capacitance C ofeach capacitor 11. All capacitors 11 are connected in series andtherefore charged with the same current “I”. In this case, capacitors 11store charges having an amount of charge “Q”, where Q=C·ΔVi=I·Δt. Thisequation can be modified into C=I·Δt/ΔVi. Since all capacitors 11 havethe same current “I”, it is obvious that the reciprocal of theinclination ΔVi/Δt in FIG. 2 is proportional to the capacitance C ofeach capacitor 11.

Since the capacitance C decreases with the degradation of capacitors 11,higher degraded capacitors 11 have a larger inclination ΔVi/Δt.Therefore, it is obvious that in FIG. 2, capacitor 11 having a subscript“i” of 4 has the largest inclination, and hence, most degraded of all.Thus, the balanced voltage Vri is determined according to the magnituderelation between the inclinations.

In order to determine the balanced voltage Vri, control circuit 15 firstdetermines the voltage adjustment range ΔVbi in S15. The voltageadjustment range ΔVbi indicates how much the voltage should be reducedfrom an initial balanced voltage Vro set when capacitors 11 are in theinitial state with no degradation. The initial balanced voltage Vro is,for example, 2.5V when capacitors 11 have a rated voltage of 2.5V. Theinitial balanced voltage Vro is calculated by multiplying theinclination by the specified coefficient “A”. As a result, higherdegraded capacitors 11 having a larger inclination have a larger voltageadjustment range ΔVbi. The specified coefficient “A” is a coefficientused to allow the balanced voltage Vri to be in a normal range in thenext step S17. The specified coefficient “A” is experimentallypredetermined and stored in the memory.

Next, control circuit 15 calculates the balanced voltage Vri from anequation: Vri=Vro−ΔVbi (S17). As described above, the voltage adjustmentrange ΔVbi increases with the degradation of capacitors 11. The balancedvoltage Vri, on the other hand, decreases with the degradation ofcapacitors 11 because the initial balanced voltage Vro is a constant.Thus, balanced voltage adjusting portions 13 adjust the voltage Viacross each capacitor 11 to become the balanced voltage Vri, makinghigher degraded capacitors 11 have a smaller voltage Vi acrossthemselves. As a result, higher degraded capacitors 11 are degradedslower than the other capacitors 11. The specified coefficient “A” ispredetermined and multiplied by the inclination in S15 so that thebalanced voltage Vri can be prevented from becoming too small ornegative in the equation of S17.

Next, control circuit 15 compares the balanced voltage Vri with adegradation limit Vg (S19). The degradation limit Vg, which is also anexperimentally predetermined value, represents the balanced voltage Vriwhen capacitors 11 are degraded to the limit of use. When the balancedvoltage Vri becomes equal to or less than the degradation limit Vg (YESin S19), this indicates that the electricity accumulating device cannotbe used any more. In this case, control circuit 15 transmits a signalindicative of degradation of the electricity accumulating device as aData signal to the vehicular control circuit (S21). Upon receiving theData signal, the vehicular control circuit informs the driver that theelectricity accumulating device is in a degraded state and urges him/herto repair it, and at the same time, stops charging the electricityaccumulating device. As a result, the electricity accumulating device isnot used in a degraded state, thereby providing high reliability. Afterthis, control circuit 15 terminates the subroutine of FIG. 3, andreturns to the main routine.

When the balanced voltage Vri is larger than the degradation limit Vg(NO in S19), on the other hand, this indicates that the electricityaccumulating device can be continued to be used. In this case, controlcircuit 15 adds 1 to the subscript “i” and updates the contents of thesubscript “i” (S23). Control circuit 15 then determines whether theupdated subscript “i” is equal to the value obtained by adding 1 to “n”(n=4) indicating the number of capacitors 11 (S25). When the subscript“i” is determined not to be equal to “n+1” (NO in S25), this indicatesthat the balanced voltage Vri has not been determined yet for allcapacitors 11. Therefore, the process returns to S13 to repeat thesubsequent steps.

When the subscript “i” is determined to be equal to “n+1” (YES in S25),on the other hand, this indicates that the balanced voltage Vri has beendetermined for all capacitors 11. As a result, control circuit 15terminates the subroutine of FIG. 3 and returns to the main routine.

The above-described subroutine of the flowchart of FIG. 3 is summarizedas follows.

Control circuit 15 first measures the first point of time t1 at whichthe voltage V1 i across each capacitor 11 is measured and the secondpoint of time t2 at which the voltage V2 i across each capacitor 11 ismeasured. Control circuit 15 then calculates the time difference Δt bysubtracting the first point of time t1 from the second point of time t2.Control circuit 15 then divides the absolute value ΔVi by the timedifference Δt and multiplies the result by the specified coefficient“A”, thereby calculating the voltage adjustment range ΔVbi of eachcapacitor 11. Control circuit 15 then subtracts the voltage adjustmentrange ΔVbi from the initial balanced voltage Vro, thereby determiningthe balanced voltage Vri. Thus, control circuit 15 determines thebalanced voltage Vri of each capacitor 11 according to the absolutevalue ΔVi.

After this, control circuit 15 outputs the determined balanced voltagesVri to respective balanced voltage adjusting portions 13. Each balancedvoltage adjusting portion 13 adjusts its balance switch 17 so that thevoltage Vi across the capacitor 11 connected thereto becomes thebalanced voltage Vri. In other words, when the voltage Vi across thecapacitor 11 is larger than the balanced voltage Vri, comparator 23turns on balance switch 17. As a result, the capacitor 11 is dischargedthrough balance resistor 19, and the voltage Vi across the capacitor 11is reduced. Later, when the voltage Vi across the capacitor 11 becomessubstantially equal to the balanced voltage Vri, comparator 23 turns offbalance switch 17. As a result, the discharge of the capacitor 11 isterminated, allowing the voltage Vi across the capacitor 11 to becomethe balanced voltage Vri as the target. This reduces the voltage appliedto the capacitor 11, thereby reducing the speed of the degradation ofthe capacitor 11. After this, the voltage Vi across the capacitor 11gradually decreases due to self-discharge while the vehicle is not inuse.

According to the above-described operation, when the use of the vehicleis finished, the voltage Vi (V24 in FIG. 2) across a highly degradedcapacitor 11 is reduced and the voltage Vi (V21 in FIG. 2) across a lessdegraded capacitor 11 remains high. This allows the highly degradedcapacitor 11 to be degraded slower than before and the less degradedcapacitor 11 to be degraded more rapidly than the other capacitors 11,thereby equalizing the degree of degradation of all capacitors 11.Therefore, it is less likely that the electricity accumulating device isunable to be used because only one capacitor 11 reaches the degradationlimit. As a result, the electricity accumulating device has a long life.

The voltages V1 i and V2 i across each capacitor 11 are measured duringthe non-charge-or discharge period of capacitors 11. Therefore, thevoltages V1 i and V2 i across each capacitor 11 reflect the influence ofthe capacitance C of each capacitor 11 indicated as the reciprocal ofthe inclination of each thick arrow of FIG. 2, but not the influence ofthe internal resistance R. The voltage Vi across each capacitor 11reflects the magnitude of the internal resistance R only eitherimmediately after the charge or the discharge of capacitors 11 isstarted or when the charge or the discharge of capacitors 11 iscompleted. Thus, in the present first exemplary embodiment, the degreeof degradation of all capacitors 11 can be equalized without calculatingthe internal resistance R, allowing the life of capacitors 11 to beextended by a simple operation.

With the above-described structure and operation, control circuit 15calculates the absolute value ΔVi of the difference between the voltagesV1 i and V2 i measured at different times from each other across eachcapacitor 11 during the non-charge-or-discharge period of capacitors 11,and determines the balanced voltage Vri from the absolute value ΔVi.Thus, the life of capacitors 11 in the electricity accumulating devicecan be extended by a simple operation.

In the present first exemplary embodiment, the first point of time t1corresponds to the start-up time of the vehicle, and the second point oftime t2 corresponds to the time when the use of the vehicle is finished.This ensures that the voltages V1 i and V2 i across each capacitor 11are measured while capacitors 11 are not being charged or discharged.The first and second points of time t1 and t2, however, are not limitedto the start-up time of the vehicle and the time when the use of thevehicle is finished, respectively. Provided that capacitors 11 are inthe non-charge-or-discharge period, the first and second points of timet1 and t2 may correspond to other points of time when the vehicle is inuse. In this case, however, it is necessary either to transmit a signalfrom the vehicular control circuit to control circuit 15 for the purposeof indicating that the charge-discharge circuits of capacitors 11 arenot in operation, or to connect current detection circuits in series tothe series circuits of capacitors 11.

Second Exemplary Embodiment

FIG. 4 is a flowchart showing a process for determining the balancedvoltage of each capacitor in an electricity accumulating deviceaccording to a second exemplary embodiment of the present invention. Theelectricity accumulating device of the present second exemplaryembodiment has the same structure as that of the first exemplaryembodiment shown in FIG. 1, and hence, the description thereof will beomitted. The present second exemplary embodiment is characterized by itsoperation, which will be described in detail as follows.

In FIG. 4, when the vehicle is started or in operation, control circuit15 reads the present voltage V1 i across each capacitor 11 at a time t1,which is during the non-charge-or-discharge period of capacitors 11, andstores them in a memory embedded in control circuit 15. In this case,the voltage V1 i across each capacitor 11 is subjected to temperaturecorrection in the same manner as in the first exemplary embodiment. Thetime t1 is not stored in the memory unlike in the first exemplaryembodiment.

Next, control circuit 15 reads the present voltage V2 i across eachcapacitor 11 and stores them in the memory embedded therein at a timet2. The time t2, which is later than the time t1, is during thenon-charge-or-discharge period of capacitors 11, that is, either whenthe vehicle is in use or when the use of the vehicle is finished. Thevoltage V2 i across each capacitor 11 is subjected to temperaturecorrection in the same manner as in the first exemplary embodiment. Thetime t2 is not stored in the memory unlike in the first exemplaryembodiment.

When the voltages V1 i and V2 i across each capacitor 11 are thusobtained, control circuit 15 executes the subroutine of FIG. 4. First,control circuit 15 assigns 1 to the subscript “i” (S51). Next, controlcircuit 15 calculates the absolute value ΔVi of the difference betweenthe voltages V1 i and V2 i across each capacitor 11 from an equation:ΔVi=|V2 i−V1 i| (S53). Then, control circuit 15 adds 1 to the subscript“i” and updates it (S55), and determines whether the subscript “i” hasreached the value obtained by adding 1 to “n” indicating the number ofcapacitors 11 (S57). When the subscript “i” is determined not to beequal to “n+1” (NO in S57), the process returns to S53 to repeat theoperation to calculate the absolute value ΔVi of the next capacitor 11.

When the subscript “i” is determined to be equal to “n+1” (YES in S57),control circuit 15 selects a minimum value ΔVmin from the absolutevalues ΔVi (S59). In the case shown in FIG. 2, the ΔV1 becomes theminimum value ΔVmin. Next, control circuit 15 again assigns 1 to thesubscript “i” (S61), and calculates the value of ratio Δi between eachabsolute value ΔVi and the minimum value ΔVmin from an equation:Δi=ΔVi/ΔVmin (S63). The value of ratio Δi thus calculated indicates howmuch larger is the absolute value ΔVi than the minimum value ΔVmin, andis therefore, 1 or greater. Capacitor 11 corresponding to the minimumvalue ΔVmin has a voltage ΔVi (ΔV1 in FIG. 2) across itself, which isequal to the minimum value ΔVmin. Therefore, the value of ratio Δi is 1.

As obvious from FIG. 2, capacitor 11 having a value of ratio Δi of 1 hasthe smallest inclination, and hence, is least degraded. Capacitors 11having a larger value of ratio Δi are more degraded. In FIG. 2,capacitor 11 having a subscript “i” of 4 is most degraded. Thus, theratio Δi indicates the degree of degradation of capacitors 11.

Next, control circuit 15 calculates the voltage adjustment range ΔVbi ofeach capacitor 11 having the subscript “i” from the correlation betweenthe value of ratio Δi and a voltage adjustment range ΔVb (S65). Thevoltage adjustment range ΔVbi is made to increase with the degradationof capacitors 11 as described in the first exemplary embodiment.Therefore, control circuit 15 stores the experimentally predeterminedcorrelation between the value of ratio Δi and the voltage adjustmentrange ΔVb in the memory, and determines the voltage adjustment rangeΔVbi according to the value of ratio Δi calculated in S63. Thecorrelation between the value of ratio Δi and the voltage adjustmentrange ΔVb has a positive correlation function, which is expressed in aformula using the least square method. Then, the value of ratio Δi isassigned to the formula so as to calculate the voltage adjustment rangeΔVbi of each capacitor 11. This saves the memory, compared with the casein which the correlation is stored as a data table in the memory.

Next, control circuit 15 calculates the balanced voltage Vri from anequation: Vri=Vro−ΔVbi (S67). The initial balanced voltage Vro is thesame as the rated voltage (2.5V) of capacitors 11 as in the firstexemplary embodiment. This means that higher degraded capacitors 11 havea smaller balanced voltage Vri. As a result, highly degraded capacitors11 are degraded slower than the other capacitors 11, thereby extendingthe life of capacitors 11 as a whole.

Next, control circuit 15 compares the balanced voltage Vri with adegradation limit Vg (S69). The degradation limit Vg has the samemeaning as in the first exemplary embodiment. When the balanced voltageVri becomes equal to or less than the degradation limit Vg (YES in S69),this indicates that the electricity accumulating device cannot be usedany more. In this case, control circuit 15 transmits a signal indicativeof degradation of the electricity accumulating device as a Data signalto the vehicular control circuit (S71). Then, control circuit 15terminates the subroutine of FIG. 3, and returns to the main routine.

When the balanced voltage Vri is larger than the degradation limit Vg(NO in S69), on the other hand, this indicates that the electricityaccumulating device can be continued to be used. In this case, controlcircuit 15 adds 1 to the subscript “i” and updates the contents of thesubscript “i” (S73). Control circuit 15 then determines whether theupdated subscript “i” is equal to the value obtained by adding 1 to “n”indicating the number of capacitors 11 (S75). When the subscript “i” isdetermined not to be equal to “n+1” (NO in S75), this indicates that thebalanced voltage Vri has not been determined yet for all capacitors 11.Therefore, the process returns to S63 to repeat the subsequent steps.When the subscript “i” is determined to be equal to “n+1” (YES in S75),on the other hand, this indicates that the balanced voltage Vri has beendetermined for all capacitors 11. As a result, control circuit 15terminates the subroutine of FIG. 4 and returns to the main routine.

The above-described subroutine of the flowchart of FIG. 4 is summarizedas follows.

Control circuit 15 first selects the minimum value ΔVmin from theabsolute values ΔVi. Control circuit 15 then calculates the voltageadjustment range ΔVbi of each capacitor 11 from the predeterminedcorrelation between the voltage adjustment range ΔVb and the value ofratio Δi between each absolute value ΔVi and the minimum value ΔVmin.Control circuit 15 then subtracts the voltage adjustment range ΔVbi fromthe initial balanced voltage Vro, thereby determining the balancedvoltage Vri. Thus, control circuit 15 determines the balanced voltageVri of each capacitor 11 according to the absolute value ΔVi.

After this, in the same manner as in the first exemplary embodiment,balanced voltage adjusting portions 13 allow the voltage Vi across eachcapacitor 11 to become the balanced voltage Vri thus determined. Thisreduces the voltage applied to highly degraded capacitors 11 so as tomake them degraded more slowly, thereby equalizing the degree ofdegradation of all capacitors 11. As a result, the electricityaccumulating device has a long life.

With the above-described structure and operation, control circuit 15calculates the absolute value ΔVi of the difference between the voltagesV1 i and V2 i measured at different times from each other across eachcapacitor 11 during the non-charge-or-discharge period of capacitors 11,and determines the balanced voltage Vri from the ratio Δi between eachabsolute value ΔVi and the minimum value ΔVmin. This eliminates the needto measure the first and second points of time t1 and t2 unlike in thefirst exemplary embodiment. Thus, the life of capacitors 11 in theelectricity accumulating device can be extended by a simple operation.

In the first and the second exemplary embodiments, control circuit 15outputs the signal indicative of degradation when the balanced voltageVri becomes equal to or less than the degradation limit Vg. This signalmay alternatively be outputted when the absolute value ΔVi becomes equalto or more than a degradation upper limit ΔVg. The degradation upperlimit ΔVg represents the absolute value ΔVi when the electricityaccumulating device cannot be used any more, and may be predeterminedand stored in the memory in control circuit 15. The absolute value ΔViincreases with the degradation of capacitors 11 as mentioned above.Therefore, it is when the absolute value ΔVi becomes equal to or morethan the degradation upper limit ΔVg, as opposed to when the balancedvoltage Vri becomes equal to or less than the degradation limit Vg thatcapacitors 11 are considered to be degraded. This makes it possible todetermine the degradation of the electricity accumulating device at theearliest possible time particularly in the second exemplary embodiment.The signal indicative of degradation may alternatively be outputted whenthese two determinations are both performed and at least one of them issatisfied, thereby improving the accuracy of determining degradation.

In the first and the second exemplary embodiments, the degradation limitVg or the degradation upper limit ΔVg may be applied in two steps. Inthe first step, the vehicular control circuit may issue a warning to thedriver and also control to limit the charging current. In the secondstep, the vehicular control circuit may control to stop charging to theelectricity accumulating device as well as issuing the warning to thedriver. As a result, it is much less likely that the electricityaccumulating device is continued to be used after being degraded.

Third Exemplary Embodiment

FIG. 5 is a graph showing the change in the voltage across eachcapacitor from time t1 to time t2 in an electricity accumulating deviceaccording to a third exemplary embodiment of the present invention.

The electricity accumulating device of the present third exemplaryembodiment has the same structure as that of the first exemplaryembodiment shown in FIG. 1, and hence, the description thereof will beomitted. The present third exemplary embodiment is characterized by itsoperation, which will be described in detail as follows. In FIG. 5, thehorizontal axis represents the time “t”, and the vertical axisrepresents the voltage Vi across each capacitor 11. In the case of ahybrid vehicle, several hundred capacitors 11 are connected in series asmentioned above, however, in the following description, only fourcapacitors 11 are connected in series for easier explanation as in thefirst exemplary embodiment. Thus, the number “n” of capacitors 11 is 4,and the subscript “i” is in the range of 1 to 4.

First assume that at the time t1 in FIG. 5, the ignition switch (notshown) of the vehicle is turned on and the vehicle is started. Controlcircuit 15 recognizes the start of the vehicle when receiving a signalindicating that the ignition switch has been turned on as a data signal(Data) from the vehicular control circuit. Control circuit 15 mayalternatively recognize the start of the vehicle by a driving voltagewhich is supposed to be applied to control circuit 15 when the ignitionswitch is turned on.

When the vehicle is started, capacitors 11 have not been charged ordischarged. Therefore, control circuit 15 immediately reads the presentnon-charge-or-discharge-period voltage V1 i (i=1 to 4) across eachcapacitor 11 sequentially from balanced voltage adjusting portions 13,and stores them in the memory embedded therein. At the same time,control circuit 15 stores the time t1 as a first point of time t1 in thememory. This means that the first point of time t1 has been measured.These operations will be described later with FIG. 7. Capacitors 11 haverespective non-charge-or-discharge-period voltages V11 to V14 acrossthemselves, which are in a low state due to self-discharge until thetime t1 after the use of the vehicle is finished last time. Thenon-charge-or-discharge-period voltages V11 to V14 are different fromeach other due to variations in the characteristics and the degree ofdegradation in all capacitors 11.

The term “non-charge-or-discharge period” is defined as a period inwhich capacitors 11 are not being aggressively charged or discharged bythe charge-discharge circuits (not shown). In other words, thenon-charge-or-discharge period includes not only a state in which nocurrent is being supplied to capacitors 11, but also a state in which aslight leakage current is flowing to capacitors 11 although thecharge-discharge circuits is not being operated.

The next time the vehicle is in use, capacitors 11 are charged withregenerative electric power whenever the brakes are applied, making thevoltage Vi across each capacitor 11 increase with time. The change inthe voltage Vi across each capacitor 11 with time is not shown in detailin FIG. 5. While capacitors 11 are continuously being charged after themeasurement of the non-charge-or-discharge-period voltage V1 i acrosseach capacitor (time t2), control circuit 15 measures acharge-or-discharge-period voltage V2 i (i=1 to 4) across each capacitor11 by using balanced voltage adjusting portions 13, and stores them inthe memory. At the same time, control circuit 15 stores the time t2 as asecond point of time t2. This means that the second point of time t2 hasbeen measured. These operations will be also described later with FIG.7. Capacitors 11 are charged with the regenerative electric powergenerated at the time of braking. Therefore, at the time t2 of FIG. 5,each capacitor 11 has a charge-or-discharge-period voltage V2 i acrossthemselves, which is larger than the non-charge-or-discharge-periodvoltage V1 i across themselves at the time t1.

The voltage Vi across each capacitor 11 changes with temperature, andtherefore, control circuit 15 stores the predetermined temperaturedependence of the voltage Vi. Control circuit 15 then corrects thenon-charge-or-discharge-period voltage V1 i and thecharge-or-discharge-period voltage V2 i across each capacitor 11according to the temperature T from temperature sensor 25.

More specifically, control circuit 15 determines the temperaturedependence of the voltage Vi across each capacitor 11 when thetemperature T is changed after capacitors 11 have been charged to aknown voltage at a reference temperature To (for example, 25° C.).Control circuit 15 determines the temperature dependence in every knownvoltage range (for example, 0.1V) until the known voltage reaches arated voltage (for example, 2.5V) of capacitors 11. The temperaturedependence of the voltage Vi across each capacitor 11 is repeatedlydetermined until capacitors 11 reach the rated voltage (2.5V) whilechanging the temperature T. The temperature T can be changed at thefollowing times: when capacitors 11 have been charged first to 0.1V,next to 0.2V, then to 0.3V, and so on at the reference temperature To(25° C.). The temperature dependences thus obtained are previouslystored in the memory of control circuit 15.

When the temperature T and a voltage Vi across capacitor 11 are thusobtained, control circuit 15 selects the temperature dependencecorresponding to the voltage Vi at the temperature T from among thetemperature dependences. Then, control circuit 15 determines the voltageVi at the reference temperature To, based on the selected temperaturedependence. The voltage Vi thus determined becomes atemperature-corrected value.

Therefore, even when the temperatures are different at the times t1 andt2 of FIG. 5, the voltage Vi across capacitor 11 at the temperature T iscorrected to the voltage Vi across capacitor 11 at the referencetemperature To, thereby improving the accuracy of calculating of thebalanced voltage Vri, which will be described later with FIG. 8. Thetemperature correction can reduce the speed of the degradation ofcapacitors 11 (which will be described in detail later) with highaccuracy, contributing to the long life of capacitors 11.

The magnitude relation between the charge-or-discharge period voltagesV21 to V24 across respective capacitors 11 is not necessarily the sameas the magnitude relation between the non-charge-or-discharge-periodvoltages V11 to V14 across respective capacitors 11, but may be theother way around according to the variations in the characteristics orthe degree of degradation in all capacitors 11.

More specifically, in FIG. 5, capacitor 11 having the largestnon-charge-or-discharge-period voltage V11 across itself at the time t1has the smallest charge-or-discharge period voltage V21 at the time t2.Capacitor 11 having the smallest non-charge-or-discharge-period voltageV14 across itself at the time t1 has the largest charge-or-dischargeperiod voltage V24 at the time t2. Therefore, in the present thirdexemplary embodiment, the balanced voltage Vri of each capacitor 11 isdetermined based on the inclination of each thick arrow shown in FIG. 5,which is obtained from the voltages V1 i and V2 across each capacitor 11measured at the times t1 and t2.

The following is a description of how the first and second points oftime t1 and t2 are determined.

FIG. 6 is a graph showing the change in the characteristics with time ofthe full voltage of the electricity accumulating device according to thethird exemplary embodiment. In FIG. 6, the horizontal axis representsthe time “t” and the vertical axis represents the full voltage Vc ofcapacitors 11.

The first point of time t1 is set at a point of time when capacitors 11are in a non-charge-or-discharge state. In FIG. 6, the first point oftime t1 is set at a point of time in the non-charge-or-discharge period,that is, when a full voltage Vc1 of capacitors 11 is substantiallyconstant. At this moment, control circuit 15 is reading thenon-charge-or-discharge-period voltage V1 i across each capacitor 11.

Next assume that at a time “ta”, regenerative electric power isgenerated by braking. As a result, capacitors 11 are charged therewith.Immediately after the start of the charge, an initial voltage riseoccurs due to the internal resistance R of all capacitors 11. A voltagerise magnitude ΔVca can be expressed by “I·R”, where “I” is a chargingcurrent to capacitors 11. As shown in FIG. 6, the voltage rise occursrapidly in an extremely short time by a time “tb”, and the full voltageVc increases with time along with the charge of capacitors 11. Controlcircuit 15 calculates the inclination of voltage ΔVc of the full voltageVc every time a predetermined time “ts” passes after the time “tb” whenthe rapid voltage rise occurs. The predetermined time “ts” ispredetermined as a time to calculate the inclination of voltage ΔVc withhigh accuracy, and is 0.1 seconds in the present third exemplaryembodiment. As apparent from FIG. 6, the inclination of voltage ΔVc iscalculated, for example, by dividing the difference ΔVcb (=Vcc−Vcb)between a full voltage Vcb at the time “tb” and a full voltage Vcc at atime “tc” by the predetermined time “ts”. The time “tc” is obtained byadding the predetermined time “ts” to the time “tb”. Since thepredetermined time “ts” is constant, the difference ΔVcb corresponds tothe inclination of voltage ΔVc. Therefore, in the following description,the inclination of voltage ΔVc means the difference between voltages(for example, ΔVcb) generated at the predetermined time “ts”.

Control circuit 15 calculates the inclination of voltage ΔVc everypredetermined time “ts” even after the time “tc” while capacitors 11 arecontinuously being charged. More specifically, control circuit 15calculates a full voltage Vcd at a time “td” when the predetermined time“ts” has passed from the time “tc”, and calculates the inclination ofvoltage ΔVcc from an equation: ΔVcc=Vcd−Vcc.

From the time “tb” onward, as capacitors 11 are being charged with theregenerative electric power, the charging current “I” to capacitors 11increases with time and reaches a maximum current. When the vehicle isslowing down and the braking is being finished, the charging current “I”continues to decrease. On the other hand, the inclination of voltage ΔVcof the full voltage Vc of capacitors 11 increases with time from thetime “tb” onward until the maximum value is reached, and then decreaseswith time. Thus, as shown in FIG. 6, the inclination of voltage ΔVccfrom the time “tc” to the time “td” is larger than the inclination ofvoltage ΔVcb from the time “tb” to the time “tc”. The inclination ofvoltage ΔVce has a maximum value from a time “te” to a time “tf”, andthe inclination of voltage ΔVcf decreases from the time “tf” to a time“tg”. Thus, the second point of time t2 represents the time (the time“tg” of FIG. 6) when the inclination of voltage ΔVcf becomes smallerthan the inclination of voltage ΔVce, that is, when the inclination ofvoltage ΔVc (ΔVcf of FIG. 6) becomes smaller than the previousinclination of voltage ΔVco (ΔVce of FIG. 6). At this moment, controlcircuit 15 is reading the charge-or-discharge-period voltage V2 i acrosseach capacitor 11. The difference between the full voltage Vc1 at thefirst point of time t1 and a full voltage Vcg at the second point oftime t2 becomes large enough. As a result, the absolute value ΔVi of thedifference between the non-charge-or-discharge-period voltage V1 i atthe first point of time t1 and the charge-or-discharge-period voltage V2i at the second point of time t2 across each capacitor 11 is larger thanthe accuracy of reading the voltages by control circuit 15, therebymaking the achieving high accuracy.

The second point of time t2 is determined while capacitors 11 are beingcharged in FIG. 6, but may alternatively be determined when capacitors11 enters a discharge state from the non-charge-or-discharge state. Inthis case, the second point of time t2 is determined in the same manneras in FIG. 6 except that the inclination of each voltage ΔVc isdetermined as an absolute value because the full voltage Vc decreaseswith time due to discharge. This indicates that the time when theinclination of voltage ΔVc has a smaller absolute value than theprevious inclination of voltage ΔVco can be determined as the secondpoint of time t2.

When the second point of time t2 is set at a point of time during thecharge period or the discharge period of capacitors 11, if theinclination of voltage ΔVc has a different sign from the previousinclination of voltage ΔVco, it means that capacitors 11 have suddenlychanged from the charge state to the discharge state, or from thedischarge state to the charge state. As a result, a voltage drop or riseoccurs in the former and latter cases, respectively, due to the internalresistance R of all capacitors 11. As the second point of time t2 is setat a later point of time after the voltage drop or rise, the absolutevalue ΔVi of the difference at this moment between thecharge-or-discharge-period voltage V2 i and thenon-charge-or-discharge-period voltage V1 i across each capacitor 11decreases. Therefore, it is likely that the absolute value ΔVi becomessmaller than the accuracy of measuring the voltages and less and lessaccurate depending on the determined second point of time t2. To avoidthis, control circuit 15 determines the second point of time t2 onlywhen the inclination of voltage ΔVc has the same sign as the previousinclination of voltage ΔVco.

The above-described operation of determining the second point of time t2is summarized as follows.

Control circuit 15 calculates the inclination of voltage ΔVc of the fullvoltage Vc of series-connected capacitors 11 every predetermined time“ts” after the initial voltage rise or drop. The initial voltage rise ordrop is caused by the internal resistance R of all capacitors 11immediately after the charge or the discharge of capacitors 11 isstarted. Then, control circuit 15 determines as the second point of timet2 the time when the inclination of voltage ΔVc has the same sign as anda smaller absolute value than the previous inclination of voltage ΔVco,and measures the charge-or-discharge-period voltage V2 i across eachcapacitor 11.

The following is a description of the overall operation of determiningthe balanced voltage Vri. FIG. 7 is a flowchart showing a process fordetermining a non-charge-or-discharge-period voltage and acharge-or-discharge-period voltage across each capacitor in theelectricity accumulating device according to the third exemplaryembodiment. The flowcharts of FIGS. 7 and 8 are shown as subroutinesbecause in FIG. 7 control circuit 15 controls the operation of theelectricity accumulating device as a whole by executing differentsubroutines called from the main routine.

Control circuit 15 executes the subroutine of FIG. 7 called from themain routine in order to determine the balanced voltage Vri at regulartime intervals (for example, on the order of minutes). Determining thebalanced voltage Vri at regular time intervals allows the balancedvoltage Vri to reflect the latest state (such as the degree ofdegradation) of capacitors 11.

When the subroutine of FIG. 7 is started, control circuit 15 firstclears the previous inclination of voltage ΔVco (S111). Morespecifically, control circuit 15 assigns “0” to the previous inclinationof voltage ΔVco, which is a memory variable embedded therein. Thisoperation is expressed in an equation: ΔVco=0 shown in S111 of FIG. 7.This is defined to mean that the value (0) on the right-hand side isassigned to the variable on the left-hand side (the previous inclinationof voltage ΔVco).

Next, control circuit 15 determines whether capacitors 11 are in thenon-charge-or-discharge state (S113). To determine this, control circuit15 receives the present charge-or-discharge state of capacitors 11 as adata signal (Data) from the vehicular control circuit (not shown). Thecharge-or-discharge state can be recognized, for example, by receivingthe data signal (Data) indicating whether the vehicular control circuitis operating the charge-discharge circuits.

When capacitors 11 are determined not to be in thenon-charge-or-discharge state (NO in S113), control circuit 15terminates the subroutine of FIG. 7 without determining the balancedvoltage Vri, and returns to the main routine. Thus, control circuit 15determines the balanced voltage Vri every time capacitors 11 enter thenon-charge-or-discharge state. As a result, the balanced voltage Vri canreflect the latest state of capacitors 11 as mentioned above. Controlcircuit 15 does not determine the balanced voltage Vri when capacitors11 are not in the non-charge-or-discharge state because of the followingreason.

If the first point of time t1 is set at a point of time in the chargeperiod, in the case of FIG. 6, the first point of time t1 is later thanthe time “ta”. The second point of time t2, on the other hand, is thetime “tg” at which capacitors 11 are being charged. Therefore, both thefirst and second points of time t1 and t2 are while capacitors 11 arebeing charged. In this case, the absolute value ΔVi of the differencebetween the voltages Vi across each capacitor 11 at the first and secondpoints of time t1 and t2 shown in FIG. 5 does not include the influenceof the voltage rise caused by the internal resistance R of capacitors11, but reflects only the influence of the capacitance C (correspondingto the inclination of each thick arrow of FIG. 5). As a result, it isimpossible to determine the balanced voltage Vri including the influenceof the internal resistance R, thereby reducing the accuracy ofdetermining the balanced voltage Vri. To avoid this, in the presentthird exemplary embodiment, the first point of time t1 is set in a pointof time when capacitors 11 are in the non-charge-or-discharge state sothat the absolute value ΔVi can include the influence of the internalresistance R. This allows the balanced voltage Vri to include theinfluence of the internal resistance R, thereby improving its accuracy.

Again in S113, when capacitors 11 are determined to be in thenon-charge-or-discharge state (YES in S113), control circuit 15determines whether the use of the electricity accumulating device isfinished (S115). The use of the electricity accumulating device isregarded to be finished when the use of the vehicle is finished.Therefore, control circuit 15 can determine that the use of theelectricity accumulating device is finished by reading the state of theignition key (not shown) transmitted from the vehicular control circuit.

When the use of the electricity accumulating device is finished (YES inS115), it means that it is affirmatively determined in S113 and that theuse of the electricity accumulating device is finished with capacitors11 still in the non-charge-or-discharge state. In other words, the useof the vehicle is finished with capacitors 11 charged with theregenerative electric power generated when the brakes are applied tostop the vehicle. Since capacitors 11 are not aggressively charged ordischarged after this, it is impossible to determine the second point oftime t2. Therefore, control circuit 15 stops the measurement of thecharge-or-discharge-period voltage V2 i across each capacitor 11,terminates the subroutine of FIG. 7, and returns to the main routine.

When the use of the electricity accumulating device is determined not tobe finished (NO in S115), on the other hand, control circuit 15 readsthe non-charge-or-discharge-period voltage V1 i across each capacitor 11from balanced voltage adjusting portions 13 (S117). At the same time,control circuit 15 stores the time as the first point of time t1 (S119).

Next, control circuit 15 reads the temperature T from temperature sensor25 (S121), and corrects the non-charge-or-discharge-period voltage V1 iacross each capacitor 11 according to the temperature T (S123). Themethod for correcting the temperature has been described earlier indetail.

Next, control circuit 15 determines whether capacitors 11 are in thecharge-or-discharge state (S125). The operation for the determination isthe same as in S113. When capacitors 11 are determined not to be in thecharge-or-discharge state (NO in S125), control circuit 15 determinesagain whether the use of the electricity accumulating device is finishedat this moment (S126). When the use is determined to be finished (YES inS126), it means that the ignition key is turned off a little after thevehicle is stopped. In this case, similar to the case of “YES in S115”,control circuit 15 terminates the subroutine of FIG. 7 and returns tothe main routine. When the use of the electricity accumulating device isdetermined not to be finished (NO in S126), on the other hand, it isimpossible to determine the second point of time t2. Therefore, theprocess returns to S125 and waits until capacitors 11 enter thecharge-or-discharge state.

When capacitors 11 are determined to be in the charge-or-discharge state(YES in S125), control circuit 15 determines whether an initial waittime has passed (S127). The initial wait time is the time between thestart and the end of an initial voltage rise or drop due to the internalresistance R of all capacitors 11 immediately after the charge or thedischarge of capacitors 11 is started. The initial wait time correspondsto the period between the time “ta” and the time “tb” of FIG. 6. Whenthe initial wait time is determined not to have passed (NO in S127), theprocess returns to S127 and waits for the initial wait time to pass.

When the initial wait time is determined to have passed (YES in S127),control circuit 15 reads the full voltage Vc of capacitors 11 via theuppermost balanced voltage adjusting portion 13 in FIG. 5 (S129). Then,control circuit 15 assigns the read full voltage Vc to a previous fullvoltage Vco and updates the previous full voltage Vco (S131).

Next, control circuit 15 determines whether the predetermined time “ts”has passed (S133). The predetermined time “ts” has been describedearlier with FIG. 6. When the predetermined time “ts” is determined tohave not passed (NO in S133), the process returns to S133 and waits forthe predetermined time “ts” to pass. When the predetermined time “ts” isdetermined to have passed (YES in S133), on the other hand, controlcircuit 15 again reads the full voltage Vc of capacitors 11 (S135).Then, the inclination of voltage ΔVc described earlier with FIG. 6 iscalculated from the full voltage Vc and the previous full voltage Vco,based on an equation: ΔVc=Vc−Vco (S137).

Next, control circuit 15 forms the product F of the obtained inclinationof voltage ΔVc and the previously obtained inclination of voltage ΔVco(S139) in order to determine whether these inclinations have the samesign. When they have the same sign, the product F becomes positive;otherwise, it becomes negative. When the product F is “0”, it means thatthe previously obtained inclination of voltage ΔVco has remained “0”after being cleared in S111. In other words, it means that theinclination of voltage ΔVc has been determined for the first time afterthe execution of the subroutine of FIG. 7. Therefore, when the product Fis “0” (YES in S141), there is no previously obtained inclination ofvoltage ΔVco to be compared with the inclination of voltage ΔVc.Therefore, the process jumps to S147, which will be described later.

When the product “F is not “0” (NO in S141), on the other hand, controlcircuit 15 determines whether the product F is positive (S143). When theproduct F is determined to be negative (NO in S143), it means that theinclination of voltage ΔVc and the previously obtained inclination ofvoltage ΔVco have different signs from each other, indicating thatcharge and discharge have been suddenly reversed. In this case, controlcircuit 15 stops the measurement of the charge-or-discharge-periodvoltage V2 i across each capacitor 11, terminates the subroutine of FIG.7, and returns to the main routine. As a result, the balanced voltageVri is not updated and remains to have the same value. Control circuit15 executes the subroutine of FIG. 7 called from the main routine atregular time intervals as mentioned above. Therefore, control circuit 15can perform the operation of determining the balanced voltage Vri thenext time capacitors 11 enter the non-charge-or-discharge state.

When the product F is determined to be positive (YES in S143), on theother hand, it means that the inclination of voltage ΔVc and thepreviously obtained inclination of voltage ΔVco have the same sign. Inthis case, control circuit 15 compares the absolute value of theinclination of voltage ΔVc with that of the previously obtainedinclination of voltage ΔVco (S145). When the absolute value of theinclination of voltage ΔVc is determined to be equal to or more thanthat of the previously obtained inclination of voltage ΔVco (NO inS145), the inclination of voltage ΔVc is increasing as shown from thetime “tb” to the time “te” in FIG. 6. Therefore, it is still impossibleto determine the second point of time t2. In this case, control circuit15 assigns the inclination of voltage ΔVc to the previously obtainedinclination of voltage ΔVco and updates it (S147). The process returnsto S131 so as to repeat the operation of determining the inclination ofvoltage ΔVc after the predetermined time “ts” has passed.

When the absolute value of the inclination of voltage ΔVc is smallerthan that of the previously obtained inclination of voltage ΔVco (YES inS145), the inclination of voltage ΔVc corresponds to the state betweenthe time “te” and the time “tg” of FIG. 6. In this case, control circuit15 reads the charge-or-discharge-period voltage V2 i across eachcapacitor 11 via balanced voltage adjusting portions 13 (S149). At thesame time, control circuit 15 stores the time as the second point oftime t2 (S151). Next, control circuit 15 reads the temperature T fromtemperature sensor 25 (S153), and corrects thecharge-or-discharge-period voltage V2 i across each capacitor 11according to the temperature T (S155). The method for correcting thetemperature has been described earlier in detail.

Through the above-described operation, the first and second points oftime t1 and t2, and the non-charge-or-discharge-period voltage V1 i andthe charge-or-discharge-period voltage V2 i across each capacitor 11 areobtained. Therefore, control circuit 15 executes the subroutine of FIG.8 to determine the balanced voltage Vri (S157). The followingdescription is based on FIG. 8.

FIG. 8 is a flowchart showing a process for determining the balancedvoltage of each capacitor in the electricity accumulating deviceaccording to the third exemplary embodiment. In FIG. 8, control circuit15 assigns 1 to the variable memory “i” embedded therein (S161). Thevariable memory “i” is defined to have the same meaning as, and ishereinafter referred to as the subscript “i”.

Next, control circuit 15 calculates the absolute value ΔVi of thedifference between the non-charge-or-discharge-period voltage V1 i andthe charge-or-discharge-period voltage V2 i across each capacitor 11from an equation: ΔVi=|V2 i−V1 i| (S163). Then, control circuit 15calculates a time difference Δt by subtracting the first point of timet1 from the second point of time t2 (S164). In other words, controlcircuit 15 calculates the time difference Δt from an equation: Δt=t2−t1.Next, control circuit 15 calculates a voltage adjustment range ΔVbi ofeach capacitor 11 from the absolute value ΔVi, the time difference Δt,and a specified coefficient “A”, based on an equation: ΔVbi=A×ΔVi/Δt(S165), where ΔVi/Δt is the inclination of each thick arrow in FIG. 5.Each inclination corresponds to the reciprocal of the capacitance C ofeach capacitor 11. All capacitors 11 are connected in series andtherefore charged with the same charging current “I”. In this case,capacitors 11 store charges having an amount of charge “Q”, whereQ=C·ΔVi=I·Δt. This equation can be modified into C=I·Δt/ΔVi. Since allcapacitors 11 have the same charging current “I”, it is obvious that thereciprocal of the inclination ΔVi/Δt in FIG. 5 is proportional to thecapacitance C of each capacitor 11.

The capacitance C decreases and an internal resistance Ri of eachcapacitor 11 increases with the degradation of capacitors 11. Therefore,higher degraded capacitors 11 have a larger voltage rise magnitude ΔVcadue to the internal resistance R immediately after the charge ofcapacitors 11 is started in FIG. 6. The internal resistance R is the sumof the internal resistances Ri of all capacitors 11, and hence, theinternal resistance Ri increases with the degradation of capacitors 11.Therefore, in FIG. 5, the absolute value ΔVi is expressed as the sum ofa voltage rise caused by the internal resistance Ri of each capacitor 11and a voltage rise with time due to the charge of each capacitor 11. Theinclination ΔVi/Δt reflects the internal resistance Ri and thecapacitance C of each capacitor 11, and higher degraded capacitors 11have a larger inclination ΔVi/Δt. In FIG. 5, capacitor 11 having asubscript “i” of 4 has the largest inclination, and hence, most degradedof all. Thus, the balanced voltage Vri is determined according to themagnitude relation between the inclinations.

In order to determine the balanced voltage Vri, control circuit 15 firstdetermines the voltage adjustment range ΔVbi in S165. The voltageadjustment range ΔVbi indicates how much the voltage should be reducedfrom an initial balanced voltage Vro set when capacitors 11 are in theinitial state with no degradation. The initial balanced voltage Vro is,for example, 2.5V when capacitors 11 have a rated voltage of 2.5V. Theinitial balanced voltage Vro is calculated by multiplying theinclination by the specified coefficient “A”. As a result, higherdegraded capacitors 11 having a larger inclination have a larger voltageadjustment range ΔVbi. The specified coefficient “A” is a coefficientused to allow the balanced voltage Vri to be in a normal range in thenext step S167. The specified coefficient “A” is experimentallypredetermined and stored in the memory.

Next, control circuit 15 calculates the balanced voltage Vri from anequation: Vri=Vro−ΔVbi (S167). As described above, the voltageadjustment range ΔVbi increases with the degradation of capacitors 11.The balanced voltage Vri, on the other hand, decreases with thedegradation of capacitors 11 because the initial balanced voltage Vro isa constant. Thus, balanced voltage adjusting portions 13 adjust thevoltage Vi across each capacitor 11 to become the balanced voltage Vri,making higher degraded capacitors 11 have a smaller voltage Vi acrossthemselves. As the result, higher degraded capacitors 11 are degradedslower than the other capacitors 11, thereby extending the life ofcapacitors 11 as a whole. The specified coefficient “A” is predeterminedand multiplied by the inclination in S165 so that the balanced voltageVri can be prevented from becoming too small or negative in the equationof S167.

Next, control circuit 15 compares the balanced voltage Vri with adegradation limit Vg (S169). The degradation limit Vg, which is also anexperimentally predetermined value, represents the balanced voltage Vriwhen capacitors 11 are degraded to the limit of use. When the balancedvoltage Vri becomes equal to or less than the degradation limit Vg (YESin S169), this indicates that the electricity accumulating device cannotbe used any more. In this case, control circuit 15 transmits a signalindicative of degradation of the electricity accumulating device as aData signal to the vehicular control circuit (S171). Upon receiving theData signal, the vehicular control circuit informs the driver that theelectricity accumulating device is in a degraded state and urges him/herto repair it, and at the same time, inhibits the driver from using theelectricity accumulating device. As a result, the electricityaccumulating device is not used in a degraded state, thereby providinghigh reliability. After this, control circuit 15 terminates thesubroutine of FIG. 8, and returns to the subroutine of FIG. 7. Afterexecuting S157 (the subroutine of FIG. 8) in the subroutine of FIG. 7,control circuit 15 returns to the main routine.

When the balanced voltage Vri is larger than the degradation limit Vg(NO in S169), on the other hand, this indicates that the electricityaccumulating device can be continued to be used. In this case, controlcircuit 15 adds 1 to the subscript “i” and updates the contents of thesubscript “i” (S173). Control circuit 15 then determines whether theupdated subscript “i” is equal to the value obtained by adding 1 to “n”(n=4) indicating the number of capacitors 11 (S175). When the subscript“i” is determined not to be equal to “n+1” (NO in S175), this indicatesthat the balanced voltage Vri has not been determined yet for allcapacitors 11. Therefore, the process returns to S163 to repeat thesubsequent steps.

When the subscript “i” is determined to be equal to “n+1” (YES in S175),on the other hand, this indicates that the balanced voltage Vri has beendetermined for all capacitors 11. As a result, control circuit 15terminates the subroutine of FIG. 8 and returns to the subroutine ofFIG. 7. After executing S157 (the subroutine of FIG. 8) in thesubroutine of FIG. 7, control circuit 15 returns to the main routine.

The above-described subroutine of the flowchart of FIG. 8 is summarizedas follows.

Control circuit 15 first measures the first point of time t1 at whichthe non-charge-or-discharge-period voltage V1 i across each capacitor 11is measured and the second point of time t2 at which thecharge-or-discharge-period voltage V2 i across each capacitor 11 ismeasured. Control circuit 15 then calculates the time difference Δt bysubtracting the first point of time t1 from the second point of time t2.Control circuit 15 then divides the absolute value ΔVi by the timedifference Δt and multiplies the result by the specified coefficient“A”, thereby calculating the voltage adjustment range ΔVbi of eachcapacitor 11. Control circuit 15 then subtracts the voltage adjustmentrange ΔVbi from the initial balanced voltage Vro, thereby determiningthe balanced voltage Vri. Thus, control circuit 15 determines thebalanced voltage Vri of each capacitor 11 according to the absolutevalue ΔVi.

While the vehicle is in use, control circuit 15 executes the subroutineof FIG. 7 called from the main routine at regular time intervals.Therefore, when the conditions to determine the balanced voltage Vri aresatisfied, the balanced voltage Vri is continued to be updated. Later,when the use of the vehicle is finished, control circuit 15 outputs eachlatest balanced voltage Vri to the corresponding balanced voltageadjusting portion 13. Thus, each balanced voltage adjusting portion 13adjusts its balance switch 17 so that the voltage Vi across thecapacitor 11 connected thereto becomes the balanced voltage Vri. Inother words, when the voltage Vi across each capacitor 11 is larger thanthe balanced voltage Vri, comparator 23 turns on balance switch 17. As aresult, the capacitor 11 is discharged through balance resistor 19, andthe voltage Vi across the capacitor 11 is reduced. Later, when thevoltage Vi across the capacitor 11 becomes•substantially equal to thebalanced voltage Vri, comparator 23 turns off balance switch 17. As aresult, the discharge of the capacitor 11 is terminated, allowing thevoltage Vi across the capacitor 11 to become the balanced voltage Vri asthe target. This reduces the voltage applied to the capacitor 11,thereby reducing the speed of the degradation of the capacitor 11. Afterthis, the voltage Vi across the capacitor 11 gradually decreases due toself-discharge while the vehicle is not in use.

According to the above-described operation, when the use of the vehicleis finished, the voltage Vi (V21 in FIG. 5) across a highly degradedcapacitor 11 is reduced and the voltage Vi (V21 in FIG. 5) across a lessdegraded capacitor 11 remains high. This allows the highly degradedcapacitor 11 to be degraded slower than before and the less degradedcapacitor 11 to be degraded more rapidly than the other capacitors 11,thereby equalizing the degree of degradation of all capacitors 11.Therefore, it is less likely that the electricity accumulating device isunable to be used because only one capacitor 11 reaches the degradationlimit. As a result, the electricity accumulating device has a long life.

With the above-described structure and operation, thenon-charge-or-discharge-period voltage V1 i across each capacitor 11 ismeasured in the non-charge-or-discharge period, and thecharge-or-discharge-period voltage V2 i across each capacitor 11 ismeasured during the charge-or-discharge period of capacitors 11. Then,the absolute value ΔVi of the difference between the voltages V1 i andV2 i is calculated, and the balanced voltage Vri is determinedtherefrom. Thus, the life of capacitors 11 in the electricityaccumulating device can be extended accurately by a very simpleoperation.

In the present third exemplary embodiment, the balanced voltage Vri isdetermined when capacitors 11 are in the charge state, but mayalternatively be determined when capacitors 11 are in the dischargestate. In either case, however, it is necessary that the first point oftime t1 is set in the non-charge-or-discharge period, and the secondpoint of time t2 is set in the charge-or-discharge period.

The reason for this is as follows. Assume that the first point of timet1 is set in the charge period, and the second point of time t2, whichis later than the first point of time t1, is set in the non-chargeperiod. In this case, a voltage drop occurs due to the internalresistance Ri of each capacitor 11 when the charging is stopped andcapacitors 11 enter the non-charge state. As a result, the voltage Viacross each capacitor 11, which increases until the charging is stoppedstarts to decrease, thereby reducing the absolute value ΔVi as much asthe voltage drop. When becomes too small, the absolute value ΔVi has aninfluence on the accuracy of measuring the voltages, thereby reducingthe accuracy of the balanced voltage Vri determined based on theabsolute value ΔVi. In addition, a drop in the voltage Vi across eachcapacitor 11 caused when charging is stopped may be larger than a risein the voltage Vi across each capacitor 11 generated by charging,depending on the charging time and the size of the internal resistanceRi of each capacitor 11 determined by the degree of degradation. As aresult, the inclination ΔVi/Δt may become negative even in the chargeperiod. When the magnitude relation between the inclinations ΔVi/Δtvaries according to the charge state or the degradation state, thebalanced voltage Vri cannot be determined correctly. This is the reasonthat the first point of time t1 is set in the non-charge period, and thesecond point of time t2 is set in the charge period. As a result, theinclination ΔVi/Δt is obtained according to the sum of the voltage risewhen charging is started and the rise in the voltage Vi across eachcapacitor 11 due to the charging. This allows the accurate determinationof the balanced voltage Vri reflecting the influence of the charge stateand the degradation state. The accurate determination of the balancedvoltage Vri can be performed in the same manner in the discharge period.

Fourth Exemplary Embodiment

FIG. 9 is a flowchart showing a process for determining the balancedvoltage of each capacitor in an electricity accumulating deviceaccording to a fourth exemplary embodiment of the present invention. Theelectricity accumulating device of the present fourth exemplaryembodiment has the same structure as that of the first exemplaryembodiment shown in FIG. 1, and hence, the description thereof will beomitted. The present fourth exemplary embodiment is characterized by itsoperation, which will be described in detail as follows.

The operation in the present fourth exemplary embodiment shown in FIG. 7is basically the same as in the third exemplary embodiment. The presentfourth exemplary embodiment does not include S119 and S151 because itdoes not use the first and second points of time t1 and t2, so that itscontrol becomes easier. The present fourth exemplary embodiment differsfrom the third exemplary embodiment in the subroutine of determining thebalanced voltage Vri, which will be described later with reference toFIG. 9.

When the non-charge-or-discharge-period voltage V1 i and thecharge-or-discharge-period voltage V2 i across each capacitor 11 areobtained by the execution of the subroutine of FIG. 7, control circuit15 performs S157 of FIG. 7 so as to execute the subroutine of FIG. 9.First, control circuit 15 assigns 1 to the subscript “i” (S181). Next,control circuit 15 calculates the absolute value ΔVi of the differencebetween the non-charge-or-discharge-period voltage V1 i and thecharge-or-discharge-period voltage V2 i across each capacitor 11 from anequation: ΔVi=|V2 i−V1 i| (S183). Then, control circuit 15 adds 1 to thesubscript “i” and updates it (S185), and determines whether thesubscript “i” has reached the value obtained by adding 1 to “n”indicating the number of capacitors 11 (S187). When the subscript “i” isdetermined not to be equal to “n+1” (NO in S187), the process returns toS183 to repeat the operation to calculate the absolute value ΔVi of thenext capacitor 11.

When the subscript “i” is determined to be equal to “n+1” (YES in S187),control circuit 15 selects a minimum value ΔVmin from the absolutevalues ΔVi (S189). In the case shown in FIG. 5, the ΔV1 becomes theminimum value ΔVmin. Next, control circuit 15 again assigns 1 to thesubscript “i” (S191), and calculates the value of ratio Δi between eachabsolute value ΔVi and the minimum value ΔVmin from an equation:Δi=ΔVi/ΔVmin (S193). The value of ratio Δi thus calculated indicates howmuch larger is the absolute value ΔVi than the minimum value ΔVmin, andis therefore, 1 or greater. Capacitor 11 corresponding to the minimumvalue ΔVmin has a voltage ΔVi (ΔV1 in FIG. 5) across itself, which isequal to the minimum value ΔVmin. Therefore, the value of ratio Δi is 1.

As obvious from FIG. 5, capacitor 11 having a value of ratio Δi of 1 hasthe smallest inclination, and hence, is least degraded. Capacitors 11having a larger value of ratio Δi are more degraded. In FIG. 5,capacitor 11 having a subscript “i” of 4 is most degraded. Thus, thevalue of ratio Δi indicates the degree of degradation of capacitors 11.

Next, control circuit 15 calculates the voltage adjustment range ΔVbi ofeach capacitor 11 having the subscript “i” from the correlation betweenthe value of ratio Δi and a voltage adjustment range ΔVb (S195). Thevoltage adjustment range ΔVbi is made to increase with the degradationof capacitors 11 as described in the third exemplary embodiment.Therefore, control circuit 15 stores the experimentally predeterminedcorrelation between the value of ratio Δi and the voltage adjustmentrange ΔVb in the memory, and determines the voltage adjustment rangeΔVbi according to the value of ratio Δi calculated in S193. Thecorrelation between the value of ratio Δi and the voltage adjustmentrange ΔVb has a positive correlation function, which is expressed in aformula using the least square method. Then, the value of ratio Δi isassigned to the formula so as to calculate the voltage adjustment rangeΔVbi of each capacitor 11. This saves the memory, compared with the casein which the correlation is stored as a data table in the memory.

Next, control circuit 15 calculates the balanced voltage Vri from anequation: Vri=Vro−ΔVbi (S197). The initial balanced voltage Vro is thesame as the rated voltage (2.5V) of capacitors 11 as in the thirdexemplary embodiment. This means that higher degraded capacitors 11 havea smaller balanced voltage Vri. As a result, highly degraded capacitors11 are degraded slower than the other capacitors 11, thereby extendingthe life of capacitors 11 as a whole.

Next, control circuit 15 compares the balanced voltage Vri with adegradation limit Vg (S199). The degradation limit Vg has the samemeaning as in the third exemplary embodiment. When the balanced voltageVri becomes equal to or less than the degradation limit Vg (YES inS199), this indicates that the electricity accumulating device cannot beused any more. In this case, control circuit 15 transmits a signalindicative of degradation of the electricity accumulating device as aData signal to the vehicular control circuit (S201). Then, controlcircuit 15 terminates the subroutine of FIG. 9, and returns to thesubroutine of FIG. 7.

When the balanced voltage Vri is larger than the degradation limit Vg(NO in S199), on the other hand, this indicates that the electricityaccumulating device can be continued to be used. In this case, controlcircuit 15 adds 1 to the subscript “i” and updates the contents of thesubscript “i” (S203). Control circuit 15 then determines whether theupdated subscript “i” is equal to the value obtained by adding 1 to “n”indicating the number of capacitors 11 (S205). When the subscript “i” isdetermined not to be equal to “n+1” (NO in S205), this indicates thatthe balanced voltage Vri has not been determined yet for all capacitors11. Therefore, the process returns to S193 to repeat the subsequentsteps. When the subscript “i” is determined to be equal to “n+1” (YES inS205), on the other hand, this indicates that the balanced voltage Vrihas been determined for all capacitors 11. As a result, control circuit15 terminates the subroutine of FIG. 9 and returns to the subroutine ofFIG. 7.

The above-described subroutine of the flowchart of FIG. 9 is summarizedas follows.

Control circuit 15 first selects the minimum value ΔVmin from theabsolute values ΔVi. Control circuit 15 then calculates the voltageadjustment range ΔVbi of each capacitor 11 from the predeterminedcorrelation between the voltage adjustment range ΔVb and the value ofratio Δi between each absolute value ΔVi and the minimum value ΔVmin.Control circuit 15 then subtracts the voltage adjustment range ΔVbi fromthe initial balanced voltage Vro, thereby determining the balancedvoltage Vri. Thus, control circuit 15 determines the balanced voltageVri of each capacitor 11 according to the absolute value ΔVi. Asdescribed in the third exemplary embodiment, the absolute value ΔVireflects the internal resistance Ri and the capacitance C of eachcapacitor 11. Therefore, in the present fourth exemplary embodiment, thebalanced voltage Vri can be determined based on the absolute value ΔVi,thereby improving the accuracy.

After this, in the same manner as in the third exemplary embodiment,balanced voltage adjusting portions 13 allow the voltage Vi across eachcapacitor 11 to become the balanced voltage Vri thus determined when theuse of the vehicle is finished. This reduces the voltage applied tohighly degraded capacitors 11 so as to make them degraded more slowly,thereby equalizing the degree of degradation of all capacitors 11. As aresult, the electricity accumulating device has a long life.

With the above-described structure and operation, thenon-charge-or-discharge-period voltage V1 i across each capacitor 11 ismeasured during the non-charge-or-discharge period, and thecharge-or-discharge-period voltage V2 i across each capacitor 11 ismeasured during the charge-or-discharge period of capacitors 11. Then,the absolute value ΔVi of the difference between the voltages V1 i andV2 i is calculated, and the balanced voltage Vri is determined from thevalue of ratio Δi between each absolute value ΔVi and the minimum valueΔVmin. This eliminates the need to measure the times t1 and t2 unlike inthe third exemplary embodiment. Thus, the life of capacitors 11 in theelectricity accumulating device can be extended accurately by a simpleoperation.

In the third and the fourth exemplary embodiments, thecharge-or-discharge-period voltage V2 i across each capacitor 11 ismeasured when the inclination of voltage ΔVc has the same sign as and asmaller absolute value than the previously obtained inclination ofvoltage ΔVco. Alternatively, the inclination of voltage ΔVc may bereplaced by a charge-discharge current “I” supplied to capacitors 11,and the previously obtained inclination of voltage ΔVco may be replacedby a previous charge-discharge current Io. The charge-discharge current“I” may be obtained by, for example, being received by control circuit15 as a data signal Data via the vehicular control circuit from thecurrent detection circuits (not shown) embedded in the charge-dischargecircuits. The charge-discharge current “I” may also be obtained byproviding current detection circuits in series to all capacitors 11 andbeing detected by the current detection circuits. In this case, in theflowchart of FIG. 7, the inclination of voltage ΔVc is replaced by thecharge-discharge current “I”, and the previously obtained inclination ofvoltage ΔVco is replaced by the previous charge-discharge current Io.Furthermore, the charge-discharge current “I” is read in S137, and thesteps related to the full voltage Vc and the previous full voltage Vco(S129, S131, and S135) are deleted. Although the current detectioncircuits are required, the operation can be more simplified than in FIG.7, and hence, the balanced voltage Vri can be determined more quickly.When the charge-discharge current “I” and the previous charge-dischargecurrent To have different signs from each other, the measurement of thecharge-or-discharge-period voltage V2 i across each capacitor 11 isstopped so as not to update the balanced voltage Vri. The balancedvoltage Vri can be determined the next time capacitors 11 enter thenon-charge-or-discharge state.

Fifth Exemplary Embodiment

FIG. 10 is a graph showing the change in the voltage across eachcapacitor from time t1 to time t2 in an electricity accumulating deviceaccording to a fifth exemplary embodiment of the present invention. Theelectricity accumulating device of the present fifth exemplaryembodiment has the same structure as that of the first exemplaryembodiment shown in FIG. 1, and hence, the description thereof will beomitted. The present fifth exemplary embodiment is characterized by itsoperation, which will be described in detail as follows. In FIG. 10, thehorizontal axis represents the time “t”, and the vertical axisrepresents the voltage Vi across each capacitor 11. In the case of ahybrid vehicle, several hundred capacitors 11 are connected in series asmentioned above, however, in the following description, only fourcapacitors 11 are connected in series for easier explanation as in thefirst exemplary embodiment. Thus, the number “n” of capacitors 11 is 4,and the subscript “i” is in the range of 1 to 4.

Assume that at the time t1 in FIG. 10, the use of the vehicle isfinished and the ignition switch (not shown) is turn off. Controlcircuit 15 recognizes the finish of the use of the vehicle whenreceiving a signal indicating that the ignition switch has been turnedoff as a data signal Data from the vehicular control circuit. At thismoment, capacitors 11 have a large end-of-use-time voltage V1 i (i=1 to4) across themselves because capacitors 11 have been charged with theregenerative electric power generated at the time of braking. As shownin FIG. 10, however, the end-of-use-time voltages V11 to V14 aredifferent from each other due to variations in the characteristics andthe degree of degradation in all capacitors 11. The time t1 is when theuse of capacitors 11 is finished and during the non-charge-or-dischargeperiod of capacitors 11. Therefore, control circuit 15 reads the presentend-of-use-time voltage V1 i (i=1 to 4) across each capacitor 11sequentially from balanced voltage adjusting portions 13, and storesthem in the memory embedded therein. At the same time, control circuit15 stores the time t1 as a first point of time t1. This means that thefirst point of time t1 has been measured.

After this, when the vehicle is not in use, capacitors 11 self-dischargeaccording to respective insulation resistance values Rzi, making thevoltage Vi across each capacitor 11 decrease with time. The insulationresistance value Rz and the capacitance C decrease with the degradationof capacitors 11. The voltage Vi across a capacitor 11 (having thesubscript “i”) causing self-discharge is expressed byVi=Voi/exp(t/(Rzi·Ci)) where “t” is the time passed until the voltage Viacross the capacitor 11 is determined from a voltage Voi across thecapacitor 11 at a certain point of time. Since the insulation resistancevalue Rzi and capacitance Ci decrease with the degradation of capacitors11 as described above, the term of exp(t/(Rzi·Ci)) increases. From this,it is obvious that the voltage Vi across the capacitor 11 decreases.Thus, the voltage Vi across the capacitor 11 becomes smaller than thevoltage Voi across the capacitor 11 at the certain point of time asdegradation proceeds, thereby increasing the inclination |Voi−Vi|/twhich is due to the change in the voltage Vi with time. Therefore,capacitor 11 having a subscript “i” of 1, which has a large absolutevalue (corresponds to |Voi−Vi|/t) of the inclination of a thick arrow inFIG. 10 is most degraded of all. The change in the voltage Vi acrosseach capacitor 11 with time is not shown in detail in FIG. 10

Assume that at the time t2, the vehicle is started next time. Controlcircuit 15 recognizes the start of the vehicle when informed that theignition switch is turned on from the vehicular control circuit. Controlcircuit 15 may alternatively recognize the start of the vehicle by adriving voltage which is supposed to be applied to control circuit 15when the ignition switch is turned on.

Since the vehicle is in the stopped state immediately after it isstarted, no regenerative electric power is generated, and hence,capacitors 11 are not charged. When the vehicle is thus started nexttime during the non-charge-or-discharge period of capacitors 11, controlcircuit 15 measures a start-up-time voltage V2 i (i=1 to 4) across eachcapacitor 11 by using balanced voltage adjusting portions 13, and storesthem in the memory. At the same time, control circuit 15 stores the timet2 as a second point of time t2. This means that the second point oftime t2 has been measured.

As is obvious from the above description, both the times t1 and t2 arein the non-charge-or-discharge period in which capacitors 11 are notbeing charged or discharged. This allows the voltages V1 i and V2 iacross each capacitor 11 to be measured in a stable state. The termnon-charge-or-discharge period” is defined as a period in whichcapacitors 11 are not being aggressively charged or discharged by thecharge-discharge circuits (not shown). In other words, thenon-charge-or-discharge period includes not only a state in which nocurrent is being supplied to capacitors 11, but also a state in which aslight leakage current is flowing to capacitors 11 although thecharge-discharge circuits are not being operated.

The voltage Vi across each capacitor 11 changes with temperature, andtherefore, control circuit 15 stores the predetermined temperaturedependence of the voltage Vi. Control circuit 15 then corrects theend-of-use-time voltage V1 i and the start-up-time voltage V2 i acrosseach capacitor 11, based on the stored temperature dependence accordingto the temperature T from temperature sensor 25.

More specifically, control circuit 15 determines the temperaturedependence of the voltage Vi across each capacitor 11 when thetemperature T is changed after capacitors 11 have been charged to aknown voltage at a reference temperature To (for example, 25° C.).Control circuit 15 determines the temperature dependence in every knownvoltage range (for example, 0.1V) until the known voltage reaches arated voltage (for example, 2.5V) of capacitors 11. The temperaturedependence of the voltage Vi across each capacitor 11 is repeatedlydetermined until capacitors 11 reach the rated voltage (2.5V) whilechanging the temperature T. The temperature T can be changed at thefollowing times: when capacitors 11 have been charged first to 0.1V,next to 0.2V, then to 0.3V, and so on at the reference temperature To(25° C.). The temperature dependences thus obtained are previouslystored in the memory of control circuit 15.

When the temperature T and a voltage Vi across capacitor 11 are thusobtained, control circuit 15 selects the temperature dependencecorresponding to the voltage Vi at the temperature T from among thetemperature dependences. Then, control circuit 15 determines the voltageVi at the reference temperature To, based on the selected temperaturedependence. The voltage Vi thus determined becomes atemperature-corrected value.

Therefore, even when the temperatures are different at the times t1 andt2 of FIG. 10, the voltage Vi across capacitor 11 at the temperature Tis corrected to the voltage Vi across capacitor 11 at the referencetemperature To, thereby improving the accuracy of calculating thebalanced voltage Vri, which will be described later with FIG. 11. Thetemperature correction can reduce the speed of the degradation ofcapacitors 11 (which will be described in detail later) with highaccuracy, contributing to the long life of capacitors 11.

The magnitude relation between the start-up-time voltages V21 to V24across respective capacitors 11 is not necessarily the same as themagnitude relation between the end-of-use-time voltages V11 to V14across respective capacitors 11, but may be the other way aroundaccording to the variations in the characteristics or the degree ofdegradation in all capacitors 11.

More specifically, in FIG. 10, capacitor 11 having the largestend-of-use-time voltage V11 across itself at the time t1 has thesmallest start-up-time voltage V21 at the time t2, and capacitor 11having the smallest end-of-use-time voltage V14 across itself at thetime t1 has the largest start-up-time voltage V24 at the time t2. Thereason for this is that as described above, the inclination Vi/tincreases as the insulation resistance value Rz and the capacitance Cdecrease with the degradation of capacitors 11. Thus, it is obvious thatcapacitor 11 having a subscript “i” of 1 is degraded of all, andcapacitor 11 having a subscript “i” of 4 is least degraded of all.Therefore, the present fifth exemplary embodiment focuses attention onthe fact that the inclination of each thick arrow of FIG. 10 obtainedfrom the end-of-use-time voltage V1 i at the first point of time t1 andthe start-up-time voltage V2 i at the second point of time t2 reflectsthe insulation resistance value Rz and the capacitance C. As a result,the balanced voltage Vri of each capacitor 11 is determined based on theabove-described inclination.

The following is a specific description of how the balanced voltage Vriis determined.

FIG. 11 is a flowchart showing a process for determining the balancedvoltage of each capacitor in the electricity accumulating deviceaccording to the fifth exemplary embodiment. The flowchart of FIG. 11 isshown as a subroutine because control circuit 15 controls the operationof the electricity accumulating device as a whole by executing differentsubroutines called from the main routine.

Control circuit 15 executes the subroutine of FIG. 11 at the time t2when the first and second points of time t1 and t2, the end-of-use-timevoltage V1 i and the start-up-time voltage V2 i across each capacitor 11are all obtained. First, control circuit 15 assigns 1 to a variablememory “i” embedded therein (S311). The variable memory “i” is definedto have the same meaning as, and is hereinafter referred to as thesubscript “i”. S311 in the flowchart shows “i=1”, which is defined tomean that the value on the right-hand side is assigned to the variableon the left-hand side in the following description. Therefore, in S311,“1”, which is the value on the right-hand side is assigned to thesubscript “i”, which is the variable on the left-hand side.

Next, control circuit 15 calculates the absolute value ΔVi of thedifference between the end-of-use-time voltage V1 i and thestart-up-time voltage V2 i across each capacitor 11 from an equation:ΔVi=|V2 i−V1 i| (S313). Then, control circuit 15 calculates a timedifference Δt by subtracting the first point of time t1 from the secondpoint of time t2 (S314). In other words, control circuit 15 calculatesthe time difference Δt from an equation: Δt=t2−t1.

Next, control circuit 15 calculates an inclination ΔVi/Δt obtained bydividing the absolute value ΔVi by the time difference Δt. The ΔVicorresponds to ΔV1 to ΔV4 of FIG. 10, and the inclination ΔVi/Δtcorresponds to the inclination of each thick arrow of FIG. 10. Then,control circuit 15 calculates the balanced voltage Vri of each capacitor11 from the predetermined correlation between the inclination ΔVi/Δt anda balanced voltage Vr (S315).

The balanced voltage Vr is a value determined according to the degree ofdegradation of capacitors 11. For example, when bland-new capacitors 11have a rated voltage of 2.5V, the balanced voltage Vr is set to 2.5V.When capacitors 11 are degraded, the balanced voltage Vri is decreasedso as to reduce the voltage applied to capacitors 11, thereby reducingthe speed of the degradation of capacitors 11. Therefore, the balancedvoltage Vr is made smaller than 2.5V according to an increase in theinclination ΔVi/Δt due to the degradation of capacitors 11. Theinclination ΔVi/Δt and the balanced voltage Vr have a nonlinearcorrelation.

The reason for this is as follows.

As described above, the insulation resistance value Rz and capacitance Creduce with the degradation of capacitors 11. The degree of theirreduction, however, cannot be expressed by a simple function that isuniquely determined with respect to the degree of degradation. Whenbrand-new capacitors 11 are degraded to some degree, the insulationresistance value Rz is reduced. After this, however, the degradation ofcapacitors 11 does not proceed so much even when the insulationresistance value Rz continues to decrease. When the insulationresistance value Rz is much more reduced, the degradation of capacitors11 proceeds at an accelerated rate, and then again slows down. Inaddition to this property, the inclination ΔVi/Δt is affected by thechange in the capacitance C along with the degradation, thus making itimpossible to uniquely determine the correlation between the inclinationΔVi/Δt and the balanced voltage Vr indicating the degree of degradation(decreasing with the degradation). The correlation varies depending onthe internal structure, the shape, or the like of capacitors 11.Therefore, their correlation is experimentally predetermined and storedin the memory embedded in control circuit 15.

When the balanced voltage Vri is determined, control circuit 15 adds 1to the subscript “i” and updates the contents of the subscript “i”(S317). Control circuit 15 then determines whether the updated subscript“i” is equal to the value obtained by adding 1 to “n” (n=4) indicatingthe number of capacitors 11 (S319). When the subscript “i” is determinednot to be equal to “n+1” (NO in S319), this indicates that the balancedvoltage Vri has not been determined yet for all capacitors 11.Therefore, the process returns to S313 to repeat the subsequent steps.

When the subscript “i” is determined to be equal to “n+1” (YES in S319),on the other hand, this indicates that the balanced voltage Vri has beendetermined for all capacitors 11. As a result, control circuit 15assigns 1 to the subscript “i” (S321), thereby updating the balancedvoltage Vri (S323).

The following is a description of how the balanced voltage Vri isupdated. The final balanced voltage Vri of each capacitor 11 isdetermined according to the ratio between the balanced voltages Vriobtained in S315. More specifically, assuming that capacitor 11 having asubscript “i” of 4 is nearly brand new as shown in FIG. 10, the balancedvoltage Vr4 is 2.5V. Next assume that capacitors 11 having a subscript“i” of 2 and 3, respectively, are slightly degraded and that theirbalanced voltages Vr2 and Vr3 are 2.45V each. Then assume that capacitor11 having a subscript “i” of 1 is most degraded and that its balancedvoltage Vr1 is 2.4V. Thus, the ratio between the voltages acrosscapacitors 11 themselves having a subscript “i” of 1 to 4 is set at2.4:2.45:2.45:2.5.

Capacitors 11 have a full charge voltage Vf of 10V, which is obtained bymultiplying the rated voltage (2.5V) of each capacitor 11 by the numberof capacitors 11 connected in series (4 in the present fifth exemplaryembodiment). As a result, the balanced voltage Vri is determined in sucha manner that the full charge voltage Vf is 10V, and that the ratiobetween the voltages across capacitors 11 themselves having a subscript“i” of 1 to 4 is 2.4:2.45:2.45:2.5. In other words, the balanced voltageVri of each capacitor 11 can be updated by dividing it by the balancedvoltage sum ΣVri, which is their sum, and multiplying the result by thefull charge voltage Vf of capacitors 11. This operation can be expressedby an equation: Vri=Vf·Vri/ΣVri. The Σ is in the range of 1 to “n” (4 inthe present fifth exemplary embodiment).

When the final balanced voltages Vr1 to Vr4 of respective capacitors 11are calculated by assigning the above values to the equation, theresults are as follows: Vr1≈2.45V, Vr2=Vr3=2.5V, Vr4≈2.55V. As a result,higher degraded capacitors 11 can have a smaller voltage acrossthemselves so as to delay the degradation. The less degraded capacitor11 having a subscript “i” of 4 has a balanced voltage Vr4, which is muchlarger than the rated voltage. As a result, this capacitor 11 is appliedwith a voltage of about 2.55V in a full charge state, thereby being moredegraded than the other capacitors 11. Determining the balanced voltageVri in this manner allows the equalization of the degree of degradationof all capacitors 11. As a result, it is less likely that theelectricity accumulating device is unable to be used because only onecapacitor 11 reaches the degradation limit. As a result, the electricityaccumulating device has a long life.

Referring again to FIG. 11, when the balanced voltage Vri is updated inS323, control circuit 15 compares the balanced voltage Vri with adegradation limit Vg (S325). The degradation limit Vg, which is also anexperimentally predetermined value, represents the balanced voltage Vriwhen capacitors 11 are degraded to the limit of use. When the balancedvoltage Vri becomes equal to or less than the degradation limit Vg (YESin S325), this indicates that the electricity accumulating device cannotbe used any more. In this case, control circuit 15 transmits a signalindicative of degradation of the electricity accumulating device as aData signal to the vehicular control circuit (S327). Upon receiving theData signal, the vehicular control circuit informs the driver that theelectricity accumulating device is in a degraded state and urges him/herto repair it, and at the same time, stops charging the electricityaccumulating device. As a result, the electricity accumulating device isnot used in a degraded state, thereby providing high reliability. Afterthis, control circuit 15 terminates the subroutine of FIG. 11, andreturns to the main routine.

When the balanced voltage Vri is larger than the degradation limit Vg(NO in S325), on the other hand, this indicates that the electricityaccumulating device can be continued to be used. In this case, controlcircuit 15 adds 1 to the subscript “i”, and updates the contents of thesubscript “i” (S329). Control circuit 15 then determines whether theupdated subscript “i” is equal to the value obtained by adding 1 to “n”indicating the number of capacitors 11 (S331). When the subscript “i” isdetermined not to be equal to “n+1” (NO in S331), this indicates thatthe balanced voltage Vri has not been determined yet for all capacitors11. Therefore, the process returns to S323 to repeat the subsequentsteps. When the subscript “i” is determined to be equal to “n+1” (YES inS331), on the other hand, this indicates that the balanced voltage Vrihas been determined for all capacitors 11. As a result, control circuit15 terminates the subroutine of FIG. 11 and returns to the main routine.

The above-described subroutine of the flowchart of FIG. 11 is summarizedas follows.

Control circuit 15 first calculates the absolute value ΔVi of thedifference between the end-of-use-time voltage V1 i and thestart-up-time voltage V2 i across each capacitor 11. Control circuit 15then measures the first point of time t1 at which the end-of-use-timevoltage V1 i is measured and the second point of time t2 at which thestart-up-time voltage V2 i is measured. Control circuit 15 thencalculates the time difference Δt by subtracting the first point of timet1 from the second point of time t2. Control circuit 15 then calculatesthe balanced voltage Vri of each capacitor 11 from the predeterminedcorrelation between the inclination ΔVi/Δt and the balanced voltage Vr.The inclination ΔVi/Δt is obtained by dividing the absolute value ΔVi bythe time difference Δt. Control circuit 15 then divides the balancedvoltage Vri of each capacitor 11 by the balanced voltage sum ΣVri, whichis their sum, and multiplies the result by the full charge voltage Vf ofcapacitors 11 so as to update the balanced voltage Vri of each capacitor11.

After this, control circuit 15 outputs the determined balanced voltagesVri to respective balanced voltage adjusting portions 13. Each balancedvoltage adjusting portion 13 adjusts its balance switch 17 so that thevoltage Vi across the capacitor 11 connected thereto becomes thebalanced voltage Vri. When the voltage Vi across the capacitor 11becomes larger than the balanced voltage Vri as a result that capacitors11 have been charged with the regenerative electric power generated bybraking, comparator 23 turns on balance switch 17. As a result, thecapacitor 11 is discharged through balance resistor 19, and the voltageVi across the capacitor 11 is reduced. Later when the voltage Vi acrossthe capacitor 11 becomes substantially equal to the balanced voltageVri, comparator 23 turns off balance switch 17. As a result, thedischarge of the capacitor 11 is terminated, allowing the voltage Viacross the capacitor 11 to become the balanced voltage Vri as thetarget. This operation equalizes the degree of degradation of allcapacitors 11, thereby extending their life.

In the present fifth exemplary embodiment, the balanced voltage Vri isupdated every time the vehicle is started. As a result, the balancedvoltage Vri is updated according to the degree of degradation of eachcapacitor 11, even if there are variations in the degree of degradationin all capacitors 11. This can equalize the degree of degradation of allcapacitors 11 with high accuracy.

With the above-described structure and operation, the end-of-use-timevoltage V1 i across each capacitor 11 is measured when the use of thevehicle is finished and in the non-charge-or-discharge period, and thestart-up-time voltage V2 i across each capacitor 11 is measured when thevehicle is started next time and during the non-charge-or-dischargeperiod of capacitors 11. Then, the absolute value ΔVi of the differencebetween the voltages V1 i and V2 i is calculated so as to determine thebalanced voltage Vri. As a result, the electricity accumulating deviceis simplified in structure and operation, and has capacitors 11 whoselife is extended accurately because of the influence of the insulationresistance value Rz and the capacitance C of each capacitor 11.

Sixth Exemplary Embodiment

FIG. 12 is a flowchart showing a process for determining the balancedvoltage of each capacitor in an electricity accumulating deviceaccording to a sixth exemplary embodiment of the present invention. Theelectricity accumulating device of the present sixth exemplaryembodiment has the same structure as that of the first exemplaryembodiment shown in FIG. 1, and hence, the description thereof will beomitted. The present sixth exemplary embodiment is characterized by itsoperation, which will be described in detail as follows.

Similar to the fifth exemplary embodiment, the end-of-use-time voltageV1 i across each capacitor 11 is measured when the use of the vehicle isfinished and in the non-charge-or-discharge period, and thestart-up-time voltage V2 i across each capacitor 11 is measured when thevehicle is started next time and during the non-charge-or-dischargeperiod of capacitors 11. In the present sixth exemplary embodiment,however, it is unnecessary to measure the first and second points oftime t1 and t2, so that its control is easier than in the fifthexemplary embodiment. The end-of-use-time voltage V1 i and thestart-up-time voltage V2 i across each capacitor 11 are corrected by thetemperature T in the same manner as in the fifth exemplary embodiment.

In this situation, control circuit 15 executes the subroutine of FIG.12. In FIG. 12, like operations are labeled with like step numerals withrespect to FIG. 11, and hence the description thereof will be omitted.

Control circuit 15 assigns 1 to the subscript “i” (S351). Then, controlcircuit 15 calculates the absolute value ΔVi of the difference betweenthe end-of-use-time voltage V1 i and the start-up-time voltage V2 iacross each capacitor 11 from an equation ΔVi=|V2 i−V1 i| (S353). Then,control circuit 15 adds 1 to the subscript “i” and updates it (S355),and determines whether the subscript “i” has reached the value obtainedby adding 1 to “n” indicating the number of capacitors 11 (S357). Whenthe subscript “i” is determined not to be equal to “n+1” (NO in S357),the process returns to S353 to repeat the operation to calculate theabsolute value ΔVi of the next capacitor 11.

When the subscript “i” is determined to be equal to “n+1” (YES in S357),control circuit 15 selects a minimum value ΔVmin from the absolutevalues ΔVi (S359). In the case shown in FIG. 10, the ΔV4 becomes theminimum value ΔVmin. Next, control circuit 15 again assigns 1 to thesubscript “i” (S361), and calculates the value of ratio Δi between eachabsolute value ΔVi and the minimum value ΔVmin based on an equation:Δi=ΔVi/ΔVmin (S363). The value of ratio Δi thus calculated indicates howmuch larger is the absolute value ΔVi than the minimum value ΔVmin, andis therefore, 1 or greater. The value of ratio Δi, which is calculatedfrom the absolute value ΔVi, reflects the insulation resistance valueRzi and the capacitance Ci of each capacitor 11 in the same manner as inthe fifth exemplary embodiment. Capacitor 11 corresponding to theminimum value ΔVmin has a voltage ΔVi across itself (ΔV4 in FIG. 10),which is equal to the minimum value ΔVmin. Therefore, Δi=1.

As obvious from FIG. 10, capacitor 11 having a value of ratio Δi of 1has the smallest inclination, and hence, is least degraded. Capacitors11 having a larger value of ratio Δi are more degraded. In FIG. 10,capacitor 11 having a subscript “i” of 1 is made degraded. Thus, thevalue of ratio Δi indicates the degree of degradation of capacitors 11.Next, control circuit 15 calculates the balanced voltage Vri of eachcapacitor 11 having the subscript “i” from the correlation between thevalue of ratio Δi and a balanced voltage Vr (S365). As described in thefifth exemplary embodiment, higher degraded capacitors 11 have a smallerbalanced voltage Vri. Therefore, control circuit 15 stores theexperimentally predetermined correlation between the value of ratio Δiand the balanced voltage Vr in the memory, and determines the balancedvoltage Vri according to the value of ratio Δi calculated in S363. Thecorrelation between the value of ratio Δi and the balanced voltage Vrcannot be uniquely determined either, and varies depending on theinternal structure, the shape, or the like of capacitors 11. Therefore,their correlation is stored as a data tabled in the memory.

Next, control circuit 15 adds 1 to the subscript “i” and updates thecontents of the subscript “i” (S367), and determines whether the updatedsubscript “i” is equal to the value obtained by adding 1 to “n”indicating the number of capacitors 11 (S369). When the subscript “i” isdetermined not to be equal to “n+1” (NO in S369), this indicates thatthe balanced voltage Vri has not been determined yet for all capacitors11. Therefore, the process returns to S363 to repeat the subsequentsteps.

When the subscript “i” is determined to be equal to “n+1” (YES in S369),on the other hand, this indicates that the balanced voltage Vri has beendetermined for all capacitors 11. As a result, control circuit 15performs the operations from the next step S321 onward, thereby updatingthe balanced voltage Vri of each capacitor 11. Since the operations fromthe next step S321 onward are the same as those from S321 onward in FIG.11, and hence, the description thereof will be omitted.

The above-described subroutine of the flowchart of FIG. 12 is summarizedas follows.

Control circuit 15 first selects the minimum value ΔVmin from theabsolute values ΔVi. Control circuit 15 then calculates the balancedvoltage Vri of each capacitor 11 from the predetermined correlationbetween the balanced voltage Vr and the value of ratio Δi between eachabsolute value ΔVi and the minimum value ΔVmin. Control circuit 15 thendivides the balanced voltage Vri of each capacitor 11 by the balancedvoltage sum ΣVri, which is their sum, and multiplies the result by thefull charge voltage Vf of capacitors 11 so as to update the balancedvoltage Vri of each capacitor 11.

When the balanced voltage Vri has been determined, balanced voltageadjusting portions 13 adjust the voltage Vi across each capacitor 11 tothe balanced voltage Vri, while capacitors 11 are being charged with theregenerative electric power generated when the vehicle is braked nexttime. This operation is the same as in the fifth exemplary embodiment.Therefore, also in the present sixth exemplary embodiment, the degree ofdegradation of all capacitors 11 can be equalized accurately, therebyextending the life of the electricity accumulating device.

With the above-described structure and operation, the end-of-use-timevoltage V1 i across each capacitor 11 is measured when the use of thevehicle is finished and in the non-charge-or-discharge period, and thestart-up-time voltage V2 i across each capacitor 11 is measured when thevehicle is started next time and during the non-charge-or-dischargeperiod of capacitors 11. Then, the absolute value ΔVi of the differencebetween the voltages V1 i and V2 i is calculated so as to determine thebalanced voltage Vri from the ratio Δi between each absolute value ΔViand the minimum value ΔVmin. This eliminates the need to measure thefirst and second points of time t1 and t2 unlike in the fifth exemplaryembodiment. Thus, the life of capacitors 11 in the electricityaccumulating device can be extended by a simple operation.

In the fifth and the sixth exemplary embodiments, control circuit 15finally determines the balanced voltage Vri by updating its value insuch a manner that the full voltage Vc becomes the full charge voltageVf. However, the updating is unnecessary if the full voltage Vc is inthe allowable input voltage range of the loads connected to theelectricity accumulating device. More specifically, assume thatcapacitor 11 having a subscript “i” of 1 has a balanced voltage Vr1 of2.5V, capacitors 11 having a subscript “i” of 2 and 3 have a balancedvoltage Vr2 and Vr3, respectively, of 2.45V, and capacitor 11 having asubscript “i” of 4 has a balanced voltage Vr4 of 2.4V as described inthe fifth exemplary embodiment. In this case, the ΣVri (=Vc) as theirsum is 9.8V, which is smaller than the full charge voltage Vf (=10V).However, in the case in which no problem arises even when the supplyvoltage to the loads is reduced to 9.8V, it is unnecessary to update thebalanced voltage Vri.

In the fifth and the sixth exemplary embodiments, the end-of-use-timevoltage V1 i across each capacitor 11 is measured when the use of thevehicle is finished and during the non-charge-or-discharge period ofcapacitors 11. The end-of-use-time voltage V1 i may alternatively bemeasured during the non-charge-or-discharge period of capacitors 11 whencapacitors 11 are discharged until the full voltage Vc reaches apredetermined discharge voltage Vd after the use of the vehicle isfinished. The predetermined discharge voltage Vd is predetermined as avoltage that little affects the degree of degradation of capacitors 11(for example, half of the full charge voltage Vf). The full voltage Vcof capacitors 11 can be discharged, for example, by reducing thebalanced voltage Vri to half and turning on balance switch 17, or byusing the charge-discharge circuits (not shown).

Controlling the discharge until the full voltage Vc reaches thepredetermined discharge voltage Vd can prevent capacitors 11 from beingapplied continuously with a voltage close to the rated voltage when thevehicle is not in use, thereby reducing the speed of the degradation ofcapacitors 11. In this case, however, the end-of-use-time voltage V1 iacross each capacitor 11 is smaller than in the fifth and the sixthexemplary embodiments, also making the absolute value ΔVi smaller. Thus,a very low predetermined discharge voltage Vd reduces the influence onthe degradation of capacitors 11, but decreases the accuracy of thebalanced voltage Vri. Therefore, the predetermined discharge voltage Vdhas an upper limit to reduce the degradation of capacitors 11. The upperlimit preferably corresponds to about half the full charge voltage Vf.

In the first to the sixth exemplary embodiments, temperature sensors 25are disposed near capacitors 11. Temperature sensors 25, however, areunnecessary such as when the electricity accumulating device is used asan emergency auxiliary power supply. This is because in such a case, thetemperature T does not change very much, making it unnecessary tocorrect the voltage Vi across each capacitor 11 according to thetemperature T.

In the first to the sixth exemplary embodiments, control circuit 15outputs the signal indicative of degradation of the electricityaccumulating device when the balanced voltage Vri becomes equal to orless than the degradation limit Vg. Alternatively, the signal may beoutputted when the absolute value ΔVi becomes equal to or more than adegradation upper limit ΔVg. The degradation upper limit ΔVg representsthe absolute value ΔVi when the electricity accumulating device cannotbe used any more, and may be predetermined and stored in the memory incontrol circuit 15. The absolute value ΔVi increases with thedegradation of capacitors 11 as mentioned above. Therefore, it isregarded as degradation when the absolute value ΔVi becomes equal to ormore than the degradation upper limit ΔVg, as opposed to when thebalanced voltage Vri becomes equal to or less than the degradation limitVg. This makes it possible to determine the degradation of theelectricity accumulating device at the earliest possible time in thesecond and the fourth to the sixth exemplary embodiments. Alternatively,the signal indicative of degradation may be outputted when both the twodeterminations are performed and at least one of them is satisfied. Thisimproves the accuracy of degradation determination.

In the first to the sixth exemplary embodiments, the degradation limitVg or the degradation upper limit ΔVg may be applied in two steps. Inthe first step, the vehicular control circuit may issue a warning to thedriver and also control to limit the charging current. In the secondstep, the vehicular control circuit may control to stop charging to theelectricity accumulating device as well as issuing the warning to thedriver. As a result, it is much less likely that the electricityaccumulating device is continued to be used after being degraded.

In the first to the sixth exemplary embodiments, capacitors 11 areelectric double layer capacitors, but may alternatively beelectrochemical capacitors or other types of capacitors.

In the first to the sixth exemplary embodiments, the electricityaccumulating device is used in a hybrid vehicle. The electricityaccumulating device can also be used in a vehicle auxiliary power supplyin various systems such as a regenerative system of a vehicle, idlingstop, electric power steering, a braking system, and an electricsupercharger. The electricity accumulating device can also be used in anapparatus in which charge and discharge are performed by capacitorsconnected in series, such as an emergency auxiliary power supply otherthan vehicles.

Industrial Applicability

The electricity accumulating device of the present invention hascapacitors whose life can be extended accurately by a simple operation,and hence, is useful particularly as an electricity accumulating vehiclehaving capacitors for storing and discharging electric power.

1. An electricity accumulating device comprising: a plurality ofcapacitors connected in series; a plurality of balanced voltageadjusting portions connected to the capacitors respectively; and acontrol circuit connected to the balanced voltage adjusting portions,wherein the control circuit performs following operations: measuring twovoltages (V1i and V2i, where “i” is 1 to “n”, where “n” represents thenumber of the capacitors) at different times from each other across eachof the capacitors during a non-charge-or-discharge period of thecapacitors by using the balanced voltage adjusting portions; calculatingan absolute value (ΔVi) of a difference between the two voltages (V1iand V2i) across each of the capacitors; measuring a first point of time(t1) at which the voltage (V1i) is measured and a second point of time(t2) at which the voltage (V2i) is measured; calculating a timedifference (Δt) by subtracting the first point of time (t1) from thesecond point of time (t2); calculating a voltage adjustment range (ΔVbi)by dividing the absolute value (ΔVi) by the time difference (Δt) andmultiplying a result of said dividing by a specified coefficient (A);determining the balanced voltage (Vri) by subtracting the voltageadjustment range (ΔVbi) from an initial balanced voltage (Vro); andcontrolling the balanced voltage adjusting portions to make a voltage(Vi) across each of the capacitors the balanced voltage (Vri).
 2. Theelectricity accumulating device of claim 1, wherein the first point oftime (t1) corresponds to a start-up time of the electricity accumulatingdevice; and the second point of time (t2) corresponds to a time when useof the electricity accumulating device is finished.
 3. The electricityaccumulating device of claim 1, wherein the control circuit performsfollowing operations: selecting a minimum value (ΔVmin) among absolutevalues (ΔVi) from a measured result to the plurality of capacitors; andcalculating the voltage adjustment range (ΔVbi) from a predeterminedcorrelation between a voltage adjustment range (ΔVb) and a value (Δi),which is the value of ratio between the absolute value (ΔVi) and theminimum value (ΔVmin).
 4. The electricity accumulating device of claim1, further comprising: a temperature sensor near the capacitors, thetemperature sensor having an output connected to the control circuit,wherein the control circuit corrects the two voltages (V1i and V2i)across each of the capacitors according to a temperature (T) obtainedfrom the temperature sensor, based on predetermined temperaturedependence of the voltage (Vi).
 5. An electricity accumulating devicecomprising: a plurality of capacitors connected in series; a pluralityof balanced voltage adjusting portions connected to the capacitorsrespectively; and a control circuit connected to the balanced voltageadjusting portions, wherein the control circuit performs followingoperations: measuring a non-charge-or-discharge-period voltage (V1i,where “i” is 1 to “n”, where “n” represents the number of thecapacitors) across each of the capacitors by using the balanced voltageadjusting portions, the voltage (V1i) being measured during anon-charge-or-discharge period of the capacitors; measuring acharge-or-discharge-period voltage (V2i) across each of the capacitorsby using the balanced voltage adjusting portions, the voltage (V2i)being measured when the capacitors are being continuously charged ordischarged after the non-charge-or-discharge-period voltage (V1i) ismeasured; calculating an absolute value (ΔVi) of a difference betweenthe non-charge-or-discharge-period voltage (V1i) and thecharge-or-discharge-period voltage (V2i); measuring a first point oftime (t1) at which the non-charge-or-discharge-period voltage (V1i) ismeasured, and a second point of time (t2) at which thecharge-or-discharge-period voltage (V2i) is measured; calculating a timedifference (Δt) by subtracting the first point of time (t1) from thesecond point of time (t2); calculating a voltage adjustment range (ΔVbi)by dividing the absolute value (ΔVi) by the time difference (Δt) andmultiplying a result of said dividing by a specified coefficient (A);determining the balanced voltage (Vri) by subtracting the voltageadjustment range (ΔVbi) from an initial balanced voltage (Vro); andcontrolling the balanced voltage adjusting portions to make a voltage(Vi) across each of the capacitors the balanced voltage (Vri).
 6. Theelectricity accumulating device of claim 5, wherein the control circuitperforms following operations: selecting a minimum value (ΔVmin) amongabsolute values (ΔVi) from a measured result to the plurality ofcapacitors; and calculating the voltage adjustment range (ΔVbi) from apredetermined correlation between a voltage adjustment range (ΔVb) and avalue (Δi), which is the value of ratio between the absolute value (ΔVi)and the minimum value (ΔVmin).
 7. The electricity accumulating device ofclaim 5, wherein the control circuit determines the balanced voltage(Vri) every time the capacitors enter a non-charge-or-discharge state.8. The electricity accumulating device of claim 5, wherein the controlcircuit calculates inclination of voltage (ΔVc) of a full voltage (Vc)of the capacitors connected in series every time a predetermined time(ts) passes after an initial voltage rise or drop, the initial voltagerise or drop being caused by an internal resistance (R) of allcapacitors immediately after charge or discharge of the capacitors isstarted; and the control circuit measures the charge-or-discharge-periodvoltage (V2i) when the inclination of voltage (ΔVc) has the same sign asand a smaller absolute value than a previous inclination of voltage(ΔVco).
 9. The electricity accumulating device of claim 5, wherein thecontrol circuit calculates the charge-or-discharge-period voltage (V2i)when a charge-discharge current (I) to be supplied from the capacitorsto the control circuit has a same sign as and a smaller absolute valuethan a previous charge-discharge current (Io) every time a predeterminedtime (ts) passes after an initial voltage rise or drop, the initialvoltage rise or drop being caused by an internal resistance (R) of allcapacitors immediately after charge or discharge of the capacitors isstarted.
 10. The electricity accumulating device of claim 5, furthercomprising: a temperature sensor near the capacitors, the temperaturesensor having an output connected to the control circuit, wherein thecontrol circuit corrects the non-charge-or-discharge-period voltage(V1i) and the charge-or-discharge-period voltage (V2i) according to atemperature (T) obtained from the temperature sensor, based onpredetermined temperature dependence of the voltage (Vi).
 11. Anelectricity accumulating device comprising: a plurality of capacitorsconnected in series; a plurality of balanced voltage adjusting portionsconnected to the capacitors respectively; and a control circuitconnected to the balanced voltage adjusting portions, wherein thecontrol circuit performs following operations: measuring anend-of-use-time voltage (V1i, where “i” is 1 to “n”, where “n”represents the number of the capacitors) across each of the capacitorsby using the balanced voltage adjusting portions, the end-of-use-timevoltage being measured when use of the electricity accumulating deviceis finished and during a non-charge-or-discharge period of thecapacitors; measuring a start-up-time voltage (V2i) across each of thecapacitors by using the balanced voltage adjusting portions, thestart-up-time voltage (V2i) being measured when the electricityaccumulating device is started next time and during thenon-charge-or-discharge period of the capacitors; calculating anabsolute value (ΔVi) of a difference between the end-of-use-time voltage(V1i) and the start-up-time voltage (V2i); measuring a first point oftime (t1) at which the end-of-use-time voltage (V1i) is measured and asecond point of time (t2) at which the start-up-time voltage (V2i) ismeasured; calculating a time difference (Δt) by subtracting the firstpoint of time (t1) from the second point of time (t2); calculating abalanced voltage (Vri) of each of the capacitors from a predeterminedcorrelation between an inclination (ΔVi/Δt) and the balanced voltage(Vr), the inclination (ΔVi/Δt) being obtained by dividing the absolutevalue (ΔVi) by the time difference (Δt); and controlling the balancedvoltage adjusting portions to make a voltage (Vi) across each of thecapacitors the balanced voltage (Vri).
 12. The electricity accumulatingdevice of claim 11, wherein the control circuit performs followingoperations: selecting a minimum value (ΔVmin) among absolute values(ΔVi) from a measured result to the plurality of capacitors; andcalculating the balanced voltage (Vri) from a predetermined correlationbetween the balanced voltage (Vr) and a value (Δi), which is the valueof ratio between the absolute value (ΔVi) and the minimum value (ΔVmin).13. The electricity accumulating device of claim 11, wherein the controlcircuit updates the balanced voltage (Vri) of each of the capacitors bydividing the balanced voltage (Vri) by a balanced voltage sum (ΣVri) asa sum of all balanced voltages (Vri), and multiplying a result by a fullcharge voltage (Vf) of all capacitors.
 14. The electricity accumulatingdevice of claim 11, further comprising: a temperature sensor near thecapacitors, the temperature sensor having an output connected to thecontrol circuit, wherein the control circuit corrects theend-of-use-time voltage (V1i) and the start-up-time voltage (V2i) acrosseach of the capacitors according to a temperature (T) obtained from thetemperature sensor, based on predetermined temperature dependence of thevoltage (Vi) across each of the capacitors.
 15. The electricityaccumulating device of claim 1, wherein the control circuit outputs asignal indicative of degradation when the balanced voltage (Vri) becomesnot more than a degradation limit (Vg) or when the absolute value (ΔVi)becomes not more than a degradation upper limit (ΔVg).
 16. Theelectricity accumulating device of claim 5, wherein the control circuitoutputs a signal indicative of degradation when the balanced voltage(Vri) becomes not more than a degradation limit (Vg) or when theabsolute value (ΔVi) becomes not more than a degradation upper limit(ΔVg).
 17. The electricity accumulating device of claim 11, wherein thecontrol circuit outputs a signal indicative of degradation when thebalanced voltage (Vri) becomes not more than a degradation limit (Vg) orwhen the absolute value (ΔVi) becomes not more than a degradation upperlimit (ΔVg).